With Youtube and the Brownell's building guides it's pretty simply to assemble an AR. Especially if you buy a complete upper and are only putting together the lower. Here are some links to help you get started:
Lower build kit's:
(great deals since it includes almost everything you need to complete your stripped lower... you still need a bullet button)
Brownells assembly videos:
Your local gun store should carry stripped lowers.... just pick the rollmark you like.