Building your own computer is not only fun (for some), but also surprisingly easy. Inside a desktop computer, there are no high voltage wires, components so delicate that you can't even breathe on them, or any amount of magic, but rather parts that fit together almost as easily as Lego blocks, with no soldering required. In fact, some of these parts fit together and come apart more easily than a pre-built computer, making them easier to service or upgrade if necessary.
Speaking of parts, you get to pick exactly which ones you want, and that way you can have higher-end stuff in more important areas. For example, my main computer is very fast, but has very little storage, as I don't store much on it. Meanwhile, there's another computer nearby that is quite slow but has lots of storage.
It can also be cheaper sometimes to build your own. This ties in with the point above, since you can "cheap out" on unimportant areas. That computer with lots of storage, for example, has a motherboard pulled from a cheap Compaq "Wal-Mart special" and a CPU from an old HP that was discarded because it was too slow for Windows 7. Upgrading the storage allowed me to have a nice file server for very cheap.
So, now that you understand why it's a good idea to build your own computer, let's get started. Before you can put the thing together, you'll need to pick out your parts. The motherboard is probably the best place to start as it will influence your available choices in many other areas. The three main things you'll need to consider when selecting a motherboard are the form factor, the CPU and RAM compatibility, and what features it has. Since the form factor is the largest of these choices, it's probably best to start there.
Many form factors are available. They range all the way from Mini-ITX to ATX and beyond. The most common of these, and the one you're likely to find in a pre-built computer, is MicroATX. The MicroATX form factor provides a good balance between compactness and functionality, often offering around 4 expansion slots. The next common size down is Mini-ITX, which has historically been used when the most important aspect of a computer was its size. This has changed in recent years with smaller, higher-performance parts, and it is now quite feasible to build a gaming computer around a Mini-ITX board. They are still quite limited, however, by the reduced expansion possibilities, as most only have one expansion slot. On the other hand, ATX is commonly used when size doesn't matter. These boards can have 7 or even more expansion slots, along with room for all sorts of optional features, huge amounts of RAM, and sometimes even multiple CPUs.
Once you have chosen a form factor, it's time to decide what kind of CPU you want. It's not important to select an exact model just yet, but it is necessary to determine what kind of socket it will need. Typical sockets include AM3+, FM2, and FM2+ from AMD, and several variations of LGA sockets from Intel. Note that if you choose a very high-end CPU, it may require a special motherboard. For example, the AMD FX-9590 uses an AM3+ socket and draws 220 watts. The MSi 990FXA-GD80 motherboard, while it has an AM3+ socket, can only supply 125W to the CPU. If you try to use that CPU and motherboard together, it will fit, but the motherboard will likely fail almost immediately.
Now that you have determined what kind of motherboard you need, based on the CPU and form factor you want, it's time to determine which particular one you want. Some of them have special features that you may find important. For example, some motherboards have WLAN ("Wi-Fi") built in, and therefore don't need an expansion card or USB adapter to connect to wireless networks. On the topic of USB, most motherboards now have USB 3.0 support, but the number of ports can vary. Some lower-end models only have two USB 3.0 ports in the back and no internal connections to connect front-mounted USB 3.0 ports or devices. It's also worth checking how many USB 2.0 ports are present, as well as how many internal USB headers there are. Most people will never run out of them, but it pays to check. After comparing whichever features you want, it's time to purchase the motherboard and move on to the CPU.
Just like with motherboards, there are many CPU choices available. Thankfully, many of them have already been eliminated by your choice of motherboard. However, you do still need to know what you want, and this is hard for many people. A good place to start is by looking at the parallelism of the tasks that you are going to perform. Most programs, even today, are single-threaded. This means that they can only run on one core, no matter how many your computer has. Meanwhile, other programs are multi-threaded, meaning that they can take advantage of more than one core. The exact number can vary from program to program, so doing some research here is advised. In both cases, if two programs that do not fully use the CPU are running at the same time, the OS will schedule the threads so that as much of the CPU as possible is in use. For example, with a quad-core CPU and four single-threaded programs running, one will run on each core. Most people will not be able to use more than about two to four cores with a normal workload, but it is always a good idea to evaluate what you plan to do with the computer before deciding on a CPU. Before purchasing one, however, there are a few more considerations.
The next important decision is the clock speed. Simply put, within a series of otherwise identical CPUs, the ones with a higher clock speed will be faster. Unfortunately, between different CPUs, it's not so simple. For example, a CPU from one manufacturer running at 2.4 GHz might be just as fast as one from another manufacturer running at 3.5 GHz. There are many websites that post benchmark results for hundreds of different CPUs. These do need to be taken with a grain of salt, as benchmark results do not necessarily reflect real-world performance.
It should be noted that AMD has come up with a new device that can be used in place of a CPU on motherboards that support it. It's called an APU, and contains a CPU and GPU on one chip. In some cases, the integrated GPU can be used alongside a dedicated video card to make it run even faster. APUs, like CPUs, are available in many different speeds, ranging from just fast enough to check email, all the way up to being great for gaming. At any rate, once the CPU – or APU – has been selected, it's time to think about cooling.
CPUs make a lot of heat. There's no way around that, at least not yet, but they can certainly be kept under control using the various cooling systems on the market. While most retail (boxed) CPUs come with stock coolers in the box – which can range from horrible to quite decent – some people either want something better or buy an OEM CPU that doesn't include one. Just like the other parts, there is a huge range of aftermarket coolers available. They start simple, with small heatsinks and fans, and go all the way up to liquid cooling, thermoelectric coolers, phase-change, and beyond. At this level, it's probably best to stick with air cooling. To select a cooler, simply find the TDP (thermal design power) specification of your CPU, and compare it to the maximum the cooler allows. As long as the TDP is the same or lower, it should work. The bigger the difference, the cooler the CPU will be able to be kept. Also, make sure that the cooler you choose is capable of being mounted to the socket you chose. While most are at least somewhat universal, there are many that are not. Now, let's move on to the next problem.
OK, so a cooler has been selected, but how does the heat get from the CPU to the cooler? Despite being mounted directly on top of the CPU, small irregularities in both the cooler base and the CPU's IHS (Integrated Heat Spreader, which is that metal plate on top of it) mean that they do not contact completely. This is where thermal paste (also known as thermal grease) steps in. It has two jobs, which are to fill any gaps between the CPU and cooler base, and to conduct heat between the two. Many coolers have a thermal pad, which is a mixture of thermal paste and wax pre-applied, which will usually do the job. As a rule of thumb, if the paste or pad is a metallic gray color, it's great. If it's a dull gray, it's OK but not the best. White pads or paste are generally considered to be quite horrible, unless it says that it's the ceramic type.
The computer is going to need somewhere to store the data it's working on, running programs, and more. That's what the RAM is for. While it does need to be the correct type for the motherboard (most take DDR3, but some take DDR4 now), the speed generally doesn't matter very much. Size, however, does matter. Most people will be just fine with 4 GB, and some might even be able to get away with 2 GB. As a general rule of thumb, disk activity will decrease and the computer will run faster as more RAM is added, up to a point. Eventually, if more RAM is added, the computer won't have anything to put in it, and there will be no more speed gains. It is possible, however, to create a "RAM disk," which acts like a hard drive but stores the data in RAM. These are generally not very useful, as the data is lost when the computer is rebooted, but they can be useful for short-term high-speed storage.
Speaking of hard drives, you're going to need one of those, or at least some kind of storage. After all, when you install something or save a file, it has to be stored somewhere. The most common way of doing so is the hard drive. The hard drive is a relatively simple device, consisting of spinning platters with a magnetic coating and moving heads that can read or change the magnetic field pattern. They are by far the cheapest way to store large amounts of data, but have gained a well-deserved reputation for being slow and unreliable. There is a better way.
The SSD is a quite recent development. The basic idea is to store the data as patterns of charges in tiny capacitors, known as "flash memory." These are known for being much more reliable, and are also much faster. They have no moving parts, so they can also access any data anywhere on the drive immediately without having to wait for a head to move over to where the data is. In fact, they are so fast that the SATA interface traditionally used isn't fast enough to keep up. Even with the fastest version of SATA, modern SSDs can read data around 4 times as fast as the interface can handle it. Because of this, some SSDs now use a PCIe interface. This means that they plug into a PCIe slot instead of having a cable running to a SATA port.
While there are many ways to store data inside a computer, sometimes you might want to store it externally. There are many ways to do this, and one of the most common is the optical drive. Many types of these drives are available. Some of the lower-end ones are CD-ROM (reads CDs, and that's it) and, CD-RW (reads and writes CDs). DVD-ROM (reads CDs and DVDs), and combo (reads CDs and DVDs, burns CDs) drives are also available, by far the most popular type today is the DVD-RW drive. Most DVD-RW drives today are actually "DVD+/-RW/RAM/CD-RW" drives, meaning they can read and write DVD+RW, DVD-RW, DVD-RAM, DVD+R, DVD-R, CD-R, and CD-RW discs, as well as reading CD-ROM and DVD-ROM discs. Also common are BD-RE (or, in the long form, "BD-RE/DVD+/-RW/RAM/CD-RW") drives, which add the ability to read and write BD-R and BD-RE discs, and the ability to read BD-ROM discs. In most cases, none of these are necessary, but many people do include one just in case.
A newer way to store data externally is the SATA hot-swap bay. SATA controllers running in the newer AHCI mode can have drives added or removed while the computer is running. This is called "hot-swapping." Hot-swap bays allow the user to easily insert and remove drives from the computer while it is running. They are generally available in sizes for 3.5" drives, 2.5" drives, and USM. USM is a relatively new standard, consisting of a 2.5" drive in a standardized shell.
Well, now we have all of this data, as well as something to process it with. It's a bit hard to tell what the computer's doing, though, unless there's some kind of video output. This is where the video card steps in. The phrase "video card" is a bit of a misnomer, as there are three types, only one of which takes the shape of an expansion card. The simplest, yet slowest, type is integrated graphics. Most motherboards have this feature, and it essentially renders all of the graphics in software and outputs it through the chipset on the board. This is very cheap and simple to implement, but offers next to no performance. It's generally considered to be good enough for just about everything but gaming. If more performance is needed, using an APU instead of a CPU, as mentioned earlier, might be the way to go. The fastest way, however, is to use discrete graphics. This is the one type of video card that is actually a card. It will typically slot into a PCI or PCIe slot, and often have additional connectors for power.
We're now only one step away from having enough parts for a working computer. It's time to pick out a power supply. The power supply takes the incoming AC power and converts it to a form the computer can use. Similar to the motherboard, these come in many different form factors. The most common, by far, is the ATX or "PS2" size. Almost every computer will have that size of power supply in it. There are a few others, however, such as Antec's CPX, which is larger than ATX, and Mini-Box's PicoPSU series, which doesn't even need a power supply mounting point since it attaches directly to the motherboard. The choice of power supply form factor will depend directly on the case's form factor, so this choice is best made at the same time as the case.
One of the most difficult choices to make when purchasing a power supply is deciding what capacity is needed. The easiest way to determine what you need is to simply ask somebody more experienced online over a forum or similar. They can also help you choose a power supply with the proper set of cables, or at least one where the cables you don't need can be removed.
It is very important to put at least as much thought into the power supply as any other part of the computer, if not more. Cheap power supplies are known for being quite dangerous, both to you and the computer. Some use cheap parts inside and fail after just a few years. Others provide such horrible output quality that they will damage the rest of the computer. Many cheap units will even explode when loaded past a certain point – even before the rated power is reached!
Now that you have selected the parts to make a working computer, it's time to put it in something. That "something" is a case. Cases come in many different sizes, ranging from little tiny boxes that have room for a Mini-ITX motherboard but no power supply (remember the PicoPSU?) all the way to ATX "super towers" that can hold an ATX motherboard, or even larger, over a dozen drives, and two power supplies. Many other features vary, such as the ability to mount a power supply on the top or bottom, and some cases even allow them to be mounted upside-down. It's also important to make sure that the case has enough drive bays of the proper types to hold all of the drives you plan to use. If it has enough, but they're too big, adapters are available to convert a 5.25" bay to 3.5" or 3.5" to 2.5".
So, you've bought the case, but it mentions that it comes with such-and-such number of fans. What do you do with these? There is surprisingly much debate about this, but the general opinion now is that it's better to have a positive pressure setup. That means that more fans are pointing in than out. Most people recommend this because any gaps in the case will have air moving outwards instead of inwards, so dust doesn't get sucked in. The negative pressure arrangement, which is the other way around, works quite well for cooling. Its main weakness, however, is that any gaps in the case will pull in dust-laden air from around the computer.
With all these fans, how do you control them? With most motherboards, there are so many fan connections available that the fans can simply be plugged in. However, it is possible that there may be too many fans for the motherboard to handle. This is where fan controllers come in. Most take the form of a device that is mounted in a drive bay and has either knobs or a touchscreen on the front. The fans plug into it, and it can either be controlled manually, or in some cases, by temperature sensors plugged into it.
Of course, some people don't just want the computer to run cool; they want it to look cool too. There are lots of different ways to do this. The most common way is LED fans. These work just like normal fans, but have one or more LEDs inside to light it up. While they are commonly blue, several other colors are available as well. For those that want more light, there are many different ways to light up the inside of a computer, from cold-cathode fluorescent lights to electroluminescent wire. There are even UV-reactive cables that glow under ultraviolet light.
Well, that's all for hardware. If we stopped here, you'd have enough to get something like "DISK BOOT FAILURE: INSERT SYSTEM DISK AND PRESS ANY KEY" on the screen. If you want to network boot, this is enough, but seeing as how you're reading this, you probably don't. Therefore, you need an operating system, or OS. Like any part of a computer, there are many different choices.
The most common OS, by far, is Microsoft Windows. This OS is famous for having the best hardware support, but that is rapidly changing. On the other hand, it's not very flexible, and also tends to be somewhat buggy and unstable. The main problem with it, however, is the price. Windows tends to be very expensive, ranging from about $100 to $300, depending on the version and "edition."
A somewhat lesser-known, but still quite famous, OS is GNU/Linux, commonly shortened to just "Linux." It's known for being significantly more flexible, but its hardware support is still somewhat lacking in some areas. Being so flexible, there are many distributions (or "distros"), which are essentially different "flavors" of Linux. Some, like Ubuntu, are aimed at desktop use, while others, such as Gentoo, are aimed at servers, and another category, including some such as CentOS, is aimed at servers. There are also several, such as Debian, that themselves are flexible enough to do all three, quite possibly even at the same time. As far as price goes, almost all of these are free, not only to use, but also to modify and redistribute.
The third main OS choice is Mac OS. Normally Mac OS is only installed on Apple computers, but it is installable on normal computers with some tweaking. The result of doing so is called a "Hackintosh." This is prohibited by Apple, and therefore is technically illegal, but some people do it anyway. This route offers the least flexibility.
After picking out the parts for your new computer, it's time to put them together. After all, a pile of parts all still in their boxes, while pretty, is not very useful. Contrary to popular belief, this step is not very difficult and is often easier than the component selection. The only tools you will likely need, in fact, are a phillips-head screwdriver, pliers, and a flashlight if the room is not well-lit.
Seeing as how most other parts plug into the motherboard, it is generally a good place to start. It is generally not necessary to do anything to the motherboard prior to installation, but some CPU coolers need a special backplate to be installed on the motherboard. As these are all different, you will need to read the manual for the cooler to determine if one is necessary and how to install it. While it is easiest to do this with the motherboard out of the case, some cases provide a hole behind the motherboard that can be used to do so while it is installed.
Now that the motherboard has been prepared for installation, there is some preparation to do to the case. Since the case is metal and the motherboard has uninsulated solder joints on the back, the two cannot touch. Because of this, it needs to be mounted on standoffs. These are small pieces of metal shaped like hexagonal tubes, with a threaded hole on one end and a threaded protrusion (like a screw) on the other. These are often included with the case and can be easily installed or removed with pliers. Some cases also come with a small socket with a slot on top. If yours does, it simply fits over the standoff and can be turned with a screwdriver. Hold the motherboard up to its place, and make sure that there is a standoff installed everywhere there is a screw hole in the board. Then, remove the board and ensure that there are no standoffs installed that do not match up with a hole in the board. Some cases will have the metal bent upwards in the most common locations to the same height as a standoff. If your case has this, no standoff is needed in that hole.
The motherboard can now be installed. This is done in a total of three steps. First, remove the I/O shield from the motherboard package. This is a piece of metal with holes cut out for the rear ports. Hold it up to the ports to determine which way it goes, then press it into the hole in the case from the inside. If one is already present, remove it first by pressing it inwards. Next, place the motherboard into the case. You can set it down a bit forward of its final location, then slide it backwards so that the ports slide into the I/O shield. Finally, install one screw into each hole, screwing it into the standoff.
Next, the case needs to be plugged into the motherboard. There are typically four connections, which are the power LED (usually labeled POWER LED), disk activity LED (HDD LED), power switch (POWER SW or PWR SW), and reset switch (RESET SW). Some cases will also have a fifth connector for a small speaker (SPEAKER, SPK, BUZZER, or BUZ/BUZZ) inside the front panel. If your motherboard includes a connector block, it should have these printed on it. Plug each connector coming from the case into the appropriate position on the connector block, ensuring that the + on the case's LED connectors are on the same side as the + on the connector block. If there is no + on the connector, there will generally be two different colors of wire coming to it, in which case the wire that is not black should be connected to the +. This only matters for LEDs and not for switches or speakers. Once these are connected, plug the connector block into the motherboard's front panel header. This header is normally located near the bottom-right of the board, but the manual should say exactly where. If there was no connector block supplied or you choose not to use it, the case's connectors can be plugged directly into the header on the motherboard.
Depending on your case's features, there may be a few more connectors. These generally include USB, audio, and sometimes IEEE 1394 (FireWire). For USB and 1394, these can be plugged directly into the motherboard, assuming there are enough sockets. In the case of audio, the connector may be labeled HD AUDIO or AC'97. You will need to consult your motherboard's manual to determine which can be connected, and where. If both are available, HD AUDIO is preferred, and only one needs to be used.
Now that the motherboard is in place and the case has been connected, it's time to install some of the components that attach to it. The CPU is a great place to start. Begin by lifting the lever on the CPU socket. If you are using an LGA CPU (no pins on the CPU itself), there is a hold-down ring that will unlock and can be lifted up together with the lever. Then, remove the plastic shipping cover. On the CPU, there should be some notches around the outside. Place the CPU on the exposed bed of contacts, aligning the notches with the protrusions in the socket. Next, swing the ring back down and gently push the lever back into place.
In the case of a PGA CPU (where the CPU has pins), nothing will appear to happen when the lever is lifted. However, the socket is now ready to accept the CPU. There will generally be an arrow in one corner of the CPU and a corresponding arrow on the socket. Align these arrows, then line up the pins and gently allow the CPU to fall into the socket. You should never have to push the CPU into place - any force required means that something is wrong. After this, swing the lock lever back into place, and the CPU should lock firmly in the socket.
If you are going to use thermal paste, place a small amount on top of the CPU at this point. There has been much debate about how much is necessary, but about half the size of a pea is generally a safe bet. The goal is to cover as much of the CPU surface with a thin layer as possible without any running over the edge. It is important that none of it ends up on the motherboard, as many thermal pastes are conductive and can cause malfunctions or damage if they short things out. It is not necessary to spread out the paste, as this will happen when the cooler is installed.
It is now time to install the CPU cooler. There are two main ways that it is done, as well as several others. The method used on several Intel stock coolers as well as AMD's stock coolers for the AM1 socket is known as the push-pin method. This consists of plastic pins that are pushed into the motherboard through holes and locked either by turning them or by a smaller pin that goes through a hole in the larger pin. Meanwhile, most other AMD coolers have a metal bracket that attaches to the board and a lever that is flipped over to lock it in place. Most aftermarket coolers have very different mounting methods, and it is generally advisable to read the manual to determine how it is supposed to attach.
While stock coolers generally have fans already attached, some (such as the one for AMD's FX-9590) simply include fans and mounting hardware in the box. Many aftermarket coolers also take this approach. Similar to how there is no standard way of mounting a cooler, there is no standard way of mounting fans onto them. Some use simple screws, while others use metal clips or rubber pieces. It is, once again, advisable to read the manual, even though this step can often be handled via trial and error. Don't forget to plug the fan into the CPU fan connector on the motherboard.
With the CPU and cooler attached, it's time to move on to the RAM. This is quite possibly the simplest component. Start by locating the slots on the motherboard, which are generally next to the CPU socket. They will have small plastic clips on one or both ends. Push these outwards if they are not already. Next, decide which slots you are going to use. Some motherboards can run in a "dual channel" configuration for faster memory access if the RAM is installed in a certain set of slots, while others will run in DCT unganged mode and can benefit from multiple sticks no matter which slots they are in. In any case, after deciding which slots to use, align the blocked portion of the socket with the notch on the stick, and press it in until the clips move inwards. Repeat this process for each stick.
Another somewhat similar component that needs to be installed is the video card. First, locate the PCIe slot that you would like to use. You will generally need a PCIe x16 slot for maximum performance, and the motherboard's manual can help you determine which slots are capable of running at that speed. After deciding on one, remove the cover piece next to it from the case, and the one below it as well if you are using a double-height card. Then, if the slot has a latch lever that pivots, push it down. If it is simply a latch tab, no action is necessary. Then, push the card into the slot until it is fully seated, and secure it with the screw(s) you removed from the slot cover(s). If you are using any other PCIe or PCI cards, such as a PCIe SSD, a PCI NIC, or anything else, repeat this process on another slot of the appropriate type.
Next, install the drives. There are five major types of drives that can be put inside a computer: 5.25", 3.5", 2.5", PCIe, and M.2. The installation of PCIe devices was covered in the previous paragraph. M.2 devices can either be inserted into an M.2 slot and screwed into the board, or installed into an adapter the same way, in which case the adapter will then install into a PCIe slot like any other card. Meanwhile, 5.25" and 3.5" drives will mount into the case itself. This is different for every case, so it will be necessary to read the manual. Usually, for 5.25" drives, a small cover will come off of the front of the case, and the drive will slide in and either clip into place or be screwed in. 3.5" drives are generally installed either in a tray that slides in from the inside, or are screwed in place somewhere. 2.5" drives may be installed similarly, or in some cases, an adapter may be necessary to enable it to be placed in a 3.5" bay if the case does not allow for 2.5" devices to be installed.
Now that the drives are installed, it's time to install the data cables. This has become significantly easier in the past few years. In fact, if you are using a PCIe or M.2 SSD and no other drives, this step is not necessary at all! If you are using SATA drives, even then it's still just a matter of plugging a SATA cable into the drive on one end, and the motherboard on the other. The days of IDE/PATA and needing to configure jumpers are thankfully gone, as are those bulky and awkward ribbon cables.
Now, all of these components are going to make a lot (or a little, depending on what you chose) of hot air. This will need to go somewhere, and the best way to make it do so is to install case fans. These are some of the simplest components to install. Just screw them into the case and plug them into an available case fan connection on the motherboard.
At this point, the computer is more or less complete, but it won't do anything at all just yet. That's why it's sort of important to install the power supply. A good place to start, generally, is mounting the power supply in the case. This is a quite simple matter of placing it in the case and screwing it in from the outside. Then, connect the cables. For some power supplies, some or all of the cables will be pre-attached. For modular or semi-modular power supplies, you will need to attach cables as you go. This is a simple matter of plugging them into the power supply. On the other end, start with the motherboard. The largest connector plugs in there. It generally either has 24 pins, or is split into two smaller connectors on the same cable, one with 20 pins and the other with 4. This plugs straight in, and if it is the 24-pin type, that's it. If it is the 20+4 type, the small connector plugs into the same socket right next to it. Elsewhere on the motherboard, there will be either a 4-pin or 8-pin socket, possibly even more than one. The corresponding plug from the power supply plugs in there. If it is an 8-pin socket, ensure that you are using an EPS12V connector and not a PCIe "PEG" power connector. If your supply has colored wires, an EPS12V connector will have 4 yellow and 4 black wires, while an 8-pin PCIe connector will have 3 yellow and 5 black. If you do not have any EPS12V connectors, two 4-pin ATX12V connectors side-by-side will work.
Connectors from the power supply also need to be plugged into the video card in most cases. The card will either have a 6-pin or 8-pin socket, and sometimes even more than one. The most common is a single 6-pin connector. If your power supply has a 6+2 pin connector, leave the 2-pin piece hanging and plug the 6-pin part into the card. If the card has an 8-pin socket, you can use an 8-pin plug or both parts of a 6-pin plug.
Drives are similarly easy to connect. SATA drives will need a single SATA power connector. Ensure that the slot is the correct way around, as it has an "L" shape and could be damaged if forced on. PATA drives, as well as some case fans, fan controllers, and other accessories use 4-pin "Molex" connectors. Make sure that they are plugged in right-side-up, as they can sometimes be forced on incorrectly, which can result in major component damage.
At this point, the hardware setup is complete. Now, it's time to move on to the software. As most people prefer Windows, that's probably a good place to start. Feel free to skip ahead to another section if you prefer a different OS.
The first part of installing an OS is to get the installer running. Since there is no OS on the computer yet, it can't be run like a normal program. Instead, the computer needs to boot from the installation disc or drive instead of its hard drive. In the case of Windows, either a CD/DVD or a USB flash drive will work. In either case, simply insert the disc or flash drive, then restart the computer. It should boot automatically, but it might be necessary to manually select it in the boot menu. Either the motherboard manual or the splash screen when turning the computer on should say how to enter the boot menu. In most cases, it involves pressing a key such as delete or F11 while the splash screen is being displayed. Once the boot menu is up, it should be possible to select the boot device and start the installer.
The installation process for Windows tends to be rather simple. It will likely ask what drive you want to install it to. Pick the hard drive or SSD you would like to use. It will then install itself and reboot from the hard drive. During the initial setup, it will ask for the product key from the package. After entering it, Windows will connect to the Internet to verify the key. It will also ask a few simple questions. For example, newer versions ask for your favorite color so that they can customize things for you.
One of the weaknesses of Windows, however, is that it does not have the greatest hardware support. Therefore, it needs special programs called drivers to work with most hardware. If some part of the computer does not seem to be working, or if Windows asks for a driver for it, simply go to the website for the manufacturer of the part, then find the proper model and download and install the driver from there.
The easiest part, perhaps, is installing other software. In most cases, that either consists of inserting a disc in the optical drive and running an installer from there, or downloading the installer from a website and running it. It will prompt for several pieces of information in the process. While the defaults are perfectly fine in many cases, some installers will have options to also install other pieces of software that you may not want. These are often checked by default, so make sure that you uncheck them.
The installation process for Linux tends to be either about the same complexity or somewhat more difficult, depending on which distribution you chose. For example, Ubuntu is widely regarded as being one of the easiest to install, while Gentoo is quite difficult and Debian is about average. Since many are so easy, there is no real point in covering them here, so only the Debian installer will be covered.
Similarly to the Windows installer, a Linux installer needs to be booted into. The same CD/DVD and USB methods that work for Windows also work here. There are also, however, more ways of doing it that do not work for Windows. For example, Debian provides "network install" files that can be served via a PXE server to install without needing any discs at all. It is also possible to use the "EFI stub" function to boot the installer from any storage device that is formatted with FAT32, without even needing to wipe the drive. These are considered to be advanced methods, and it is recommended to use the CD method if possible.
Once the installer is running, it's quite simple to use. It will ask a series of questions. If you don't know what a question is asking, you can either accept the default if there is one, or ask for help online. The worst that can happen is that all writeable disks connected to the computer could be erased, and it still won't work. It is not possible to physically break the computer from the installer (and no, that's not a challenge).
The step of the installation that most new users dread most is partitioning. Sure, you can spend all day discussing and researching partitioning schemes, and then set it up manually. In fact, experienced users often prefer to set it up manually, as it gives them the control to partition the disk exactly how they want. New users, however, should probably just choose "Guided - use entire disk" and "All files in one partition." It will also ask what disk you want to install to, and prompt for you to confirm the changes. It will then erase the disk and begin installing.
Later on, it will ask what software "collections" (also known as "tasks") you want installed. Assuming you are using a network mirror (you really should), there will be a choice of several desktop environments, a few types of servers, and "standard system utilities." The standard utilities should be selected, as well as "print server" if you own a printer. I then recommend researching the desktop environment choices to see which one you like best, and choosing it as well. For example, I prefer the KDE desktop environment (out of the ones listed, anyway), so I would pick "Debian desktop environment," "... KDE," and "Standard system utilities."You can choose more than one if you aren't sure, and they can even be changed later.
Some people, however, prefer Mac OS. As stated earlier, installing Mac OS on a computer not made by Apple is prohibited. If you choose to go this route anyway, be aware that there are many different unofficial versions in circulation. Some or all of these may not work on your computer, and the installation procedure varies significantly depending on which version you choose, as well as the hardware in your computer. As a result, the installation procedure will not be covered in this guide.
To put it simply, now that the hardware and software have been selected and assembled, the computer is ready to use. When doing so, keep in mind that since you built the thing, it won't be too hard to fix if you break something while experimenting. Similarly, since more or less all desktop computers are built mostly the same, this means that you now have knowledge that can be valuable if you need to fix one of them. Anyway, what are you doing reading this now that you have a shiny new computer to play with?