Writing a post or series of posts that provide guidance about building a computer has been on my agenda, but Lifehacker did a very good job with theirs. For now, I am just going to direct you to their post:
This stuff is on the advanced side, but it far from impossible. I don’t recommend attempting it unless you have moderately strong technical background, don’t mind doing all of your own troubleshooting, and want to learn more about how computers work/ how to fix them.
In my opinion the best way to learn how to fix computers is to break them and make them better (That’s how all three of the computer geeks who write on this blog learned). This could be an excellent learning experience.