To assemble a lower, you don't need much in terms of tools. Hammer, punches, vice grips, needle nose pliers, masking tape.
If you start building uppers, you'll need an armorer's wrench, torque wrench, snap ring pliers, receiver block, and a few other things I can't think of.
Brownells has entire section of AR-specific tools. They also have instructions on assembly, as does AR15.com.