Once you've chosen your motherboard, you're ready to choose everything else. Make sure to get the CPU that's the right brand and the right pin configuration to fit your motherboard. Pick whichever CPU clock speed fits your budget and intentions. (If you purchase a motherboard/CPU combo, you can skip this step.)
What is clock speed? Essentially, it refers to how many instructions a CPU can execute in a second. For some processes, a really fast, single-core CPU may be better-suited than a slower multi-core processor. What’s best for you will depend on how you want to use your computer.
You'll need to use the RAM with the correct pin configuration that will match your motherboard. If your motherboard is using a specialty RAM configuration (normally to improve performance), make sure the RAM you buy matches its requirements. Some motherboards support RAM in pairs of memory sticks and others may require you to add three sticks at a time. Make sure you know which kind of motherboard you’re using before you buy RAM so that they match up.
If the case doesn't come with a power supply, you'll need to choose one. Make sure its connectors match the motherboard. Three hundred watts are enough for low-power machines, but if you're building a gaming machine with multiple video cards or a machine with lots of disks, you may want to consider something bigger. There are tools online that help you estimate how much power your computer will need based on the components you’re including in the machine. It’s a good idea to add another 10 percent to the power requirements. This will help guarantee your computer will have enough power and gives you the option of upgrading further down the road.
Choose a video card if you're not using the onboard video on the motherboard.
Choose an optical drive. If you are building a cheap machine, get the cheapest CD-ROM drive you can find. If you want to burn Blu-Rays, DVDs and CDs, make sure the drive can handle it.
Choose a hard disk -- or more than one. Check to see what your motherboard supports -- SATA 3.0GB/s or SATA 6GB/s. If your motherboard can support a SATA 6GB/s drive, you may want to invest in one. Most drives can run on SATA 3.0GB/s. If you want -- and your budget allows -- you can opt for a solid-state drive instead of or in addition to a hard drive. Solid-state drives take up less space, are faster and aren’t noisy but they’re also more expensive and tend to have lower storage capacity.
Choose an operating system: Microsoft’s Windows has widespread adoption, but make sure the version you buy has the features you want. There are other options -- if you prefer the Linux operating system you’ll find hundreds of variations online, some of which are free. And if you want a real challenge, you can attempt to build a hackintosh -- a non-Apple computer running the Mac operating system. But hackintoshes are notoriously tricky to build, they can be unreliable machines and you can’t expect to get any technical support from Apple.