You have a few key choices to make.
1. Decide on a "M4" or "M16" like configuration, i.e. a 16" barrel with a collapsible stock or a 20" barrel with a fixed stock.
2. Decide on a removable carry handle, no carry handle or a fixed (non-removable) carry handle. With the carry handle removed, you have a rail on which to mount either a scope of some sort or an iron sight. For flexibility purposes, I would recommend that you at least buy an AR with a removable carry handle even if you don't currently plan to install a scope.
3. Decide on a chrome-lined or non-chrome lined (i.e., chrome moly) barrel.
4. Once you have answered 1, 2 and 3 above, decide on the brand of rifle you want. Good choices to pick from are Colt, Bushmaster, Rock River, Armalite, DPMS and Stag. Each of these rifles brands likely has more choices for you to make.
5. You also need to think about what you are going to use your rifle for, i.e., do you want a military-spec or a law enforcement spec rifle?
I have a 20" Bushy (looks like an M16) and a 16" Rock River Entry Tactical (M4 style). On balance, both are great weapons. The Bushy is closer to military spec. Rock River rifles come from the factory with more goodies for the money. My RRA came from the factory with a Yankee Hill free-float quad rail, Hogue grip, and two-stage trigger. I then added an ACOG TA31 RCO scope.
For my Bushy, I free-floated the barrel and installed a Hogue grip, tuned Rock River two-stage trigger and a Leupold MR/T 3x9x36 scope.
I would encourage you to surf ar15.com, go to a few gun shows, go by the gun range and then buy with confidence. Depending how trick you want your rifle(s) to be, this can be an expensive hobby. We haven't even started talking about ammo, magazines, magpul anti-tilt followers, GI .50 caliber ammo cans, dessicants, and on and on. Good luck.