Ok, so you want to build an AR-15.
If you're THIS new, I'll advise against it. I was pretty new to AR-15;'s and tried to build my own, and ended up bugging a friend quite a bit in the end.
We're still friends, but I'm sure he wanted to beat the snot out of me after a while.
So wether you build it or buy it, here's my beginners tips:
1) Get a lower with a well known name. Rock River arms, Bushmaster, DMPS are my big three. Othres might chime in with theirs. Don't skimp on this.
2) Get a flat top upper. This means don't get an A1 or an A2 upper with an integrated carry handle. You want one with a rail across the top. You can always attach a detachable carry handle to the top of a flattop to get that "old style AR look"
3) Get a chrome lined barrel if you can. It makes cleaning your barrel SO MUCH EASIER. Don't skimp on this piece either.
4) Don't worry about free floating railed handguards, match trigger, taclatch, or fancy gas tubes while starting out, especially if you want to save money. If you plan on attaching lots of gadgets to your AR, fine, go ahead and get them, but you don't NEED them starting out.
5) Learn how to shoot your rifle with irons before you get any fancy red-dot sights.
6) grit your teeth, and get a subscription to AR15.com if youre actually going to build your own. They have lots of instructions on what tools you'll need. Try and ignore the crazyness; there ARE actually helpful people there. The loud crazies ruin the reputation of the site for everyone else.
If your'e going to assemble your own from parts, I suggest at least getting an upper with a barrel attached. That's the hardest part to assemble.
http://www.del-ton.com is where I got a lot of my parts.