I just built my first AR. I was very nervous, but it went fine. Most everything mentioned above is good advice, but to add to these; Watch the videos on Brownells, Consider buying Potterfield's video from Midway USA (it's only $15 and very informative), the pivot pin detent spring tool is a LIFE SAVER (I put my first pin in about 60 seconds w/no problems), a roll pin starter would be a good idea (You can start them w/o it, but it seems easier w/it. I'll have one b4 my next build), I don't have roll pin punches, just regular ones, and they worked fine, I just used my hammer testing block to support the trigger guard (it's made of plastic). In the words of Tiger, Just do it! and have fun!!!
Brian