Since you are new to the AR-15 I would suggest either buying a complete basic rifle and adding the parts you want or buy a separate upper and lower from a reputable company, such as Bushmaster or Rock River. With the two major components assembeled you will not need to prchase the necessary tools to build an upper, which can be costly. Plus, by buying the upper separately there may be more configurations available than there would be by purchasing the complete rifle.
Just a warning though, many of the members here have experience a slight problem. Once you get one AR, you just gotta get more. Your second or third one would be a good time to do a complete build, or at least an upper, once you get the basic operation down and understand how everying fits together and works.
Good luck and welcome to the board.
Sarge