Ok you should always start from the MOTHERBOARD.
Here's where your first choice is. Do you want to make a small computer that will fit in many tight areas, or just a regular sized computer?
Now your choices are narrowed down a little.
Then decide if you want INTEL or AMD?
Intel is generally more expensive and more well known, I guess you could say. AMD however is seeming to beat out Intel at the moment.
Now your choices are HUGELY narrowed.
After you decided that you, for example want an AMD processor, you need an AMD compatible motherboard.
Now it can get KINDA tricky, but don't worry.
There are a few different processor sockets.
A quick way to avoid an hassle, is to pick out the processor your want, and check the kind of slot it goes into. AM2, AM2+, and AM3.
Now that you have your processor, pick a motherboard that has the socket your processor goes into.
Ok now you have your dream processor and dream motherboard
Now you need RAM.
You have to look at your motherboard specifications to see what kind of RAM you can have.
Most processors are DDR2, the newer more powerful ones use DDR3, but it will say.
Now that you know what kind of RAM you can use, pick how much you need.
Vista is usually good with 4 gigs, if you're editing 8 gigs MINIMUM. Basic internet surfing and mild gaming 2 gigs will do.
Ok now you have RAM, Motherboard, and a processor.
Now you need a hard drive, some drivers, and some fans.
All motherboards come with SATA slots that you would plug your Hard drive into, so this isn't really hard. Pick something brand name like Western Digital and decide how much memory you need. 640GB will do you plenty unless, once again you edit.
Now just pick some drivers, DVD burners are basically the norm now, and it is split 50/50 on SATA (Newer)and IDE (Old Reliable).
Most motherboards come with both, so once again not a problem.
Fans are simply, pick how much you need, place them, plug them in and you're good.
I hope I didn't confuse you, ask questions if you're lost