Part 1
Why you ask. It's cheaper and you can give yourself a big pat on the back if you actually get it working!
You do not have to be a rocket scientist to build your own computer. If I can do it a chimp could!
The first thing to consider is how much you want to spend. The next is what you want to use it for.
If you want to game or trans code video on it you will need a more powerful cpu and more ram if you want to multi-task or boot up faster.
Computers consist of a few basic parts:
Optical drive
CPU-central processing unit
mobo-motherboard-the thing that you plug everything into
ram-random access memory
case-the thing that it all fits into
psu-power supply unit
hard drive-the thing that stores all the crap.
video card(if you plan on gaming)
An operating system-there are many free ones out there:
http://distrowatch.com/
Or if you want to waste your money you can always give it to Bill Gates!
Keep in mind if you want to build a Mac you need to buy the over priced parts from them!
Thats it!