Read, read, read.........and watch how to vids.............and read reviews. Reviews are (I think) important to read about any part your thinking of installing on your weapon.
So much information out there about how to build an AR. It is a lot of fun plus you learn a lot more from building one from parts than you would just buying one, and you get to put on it what you want instead of replacing parts.
Be very careful though that you don't catch the bug associated with building your own.
I did........
Stay away from cheap ingredients, and build them as though your life will depend on them. Buying top $ top of the line isn't necessary for a quality rifle or pistol, but going cheap I don't think is a good idea.
U-tube is another good place to go and watch tutorials on building one, but don't watch just one, because opinions vary greatly on how to do certain tasks.
Please take note, these are only my opinions, and opinions are like a--holes, everybody has one.
Below is only a couple of bookmarks I had that I thought worth mentioning.
Good luck and have fun.........
http://www.snipercountry.com/Articles/AR15_part1.asphttp://forums.officer.com/showthread.ph ... -AR-15-huhhttp://www.sksboards.com/smf/index.php?topic=33774.0http://www.brownells.com/.aspx/lid=1100 ... R-15_Video