I would spend several weeks reading different forums, particularly AR forums before buying anything.
Both Midway and Brownells has good vids and building your own from kit parts.
PSA is generally regarded as the best bang for the buck for a mil-spec AR.
Figure a minimum of $700 for a mil-spec gun and slightly less for non mil-spec. Used is good IF you know what you are buying. With all the hundreds of parts floating around that can be difficult. I recommend you buy a complete factory assembled AR as the first purchase, and then build after you have hands on experience.
Most AR owners are very brand loyal and well highly recommend the brand they own. That is why I recommend...
Aero Precision (parts)
Bushmaster (old ones)
I would put S&W and Windham on the list but I don't own either one. I have shot them and they seem GTG.