Congrats brat,
Before we can give you advice, you need to decide on what you want the rifle to do. I assume you are looking at 5.56 / .223 for the cartridge.
Some questions:
Is it for hunting, do you want a short barrel, for 3 gun contests, for home defense, do you live in a state where you can add a suppressor?
My personal opinion on AR's: piston driven systems are a waste of money on an AR, there are better designs to take advantage of piston drive. Ammo, 62 grain mil spec is fit for making noise, 55 gr FMJ is better and if money is no object, Hornady 55 gr GMX's are the best for defense or hunting larger critters. 50-60gr V-max's are best for varmints, coyotes and select shots on deer. The heavy bullets, 75 gr or so may need a little more twist rate, say 1-7 or 1-8. Otherwise 1-8 or 1-9 will handle everything.
Sights: what sort of iron sights do you want? If you want a scope, depending on the power, the typical M4 front sight post can be a nuisance. A flat top upper makes it easy to mount any optic you want, but you will likely want folding iron sights too.
Upper attachments / hand guard: do you want to hang light, laser, forward grip on the hand guard? That will determine which hand guard you need.
A couple of good items to consider when you have the money: a coated bolt / carrier combo is very good, this I rate as a "must have". A better / wider charging handle is also a must have, but depends if you have an optic mounted. An after market trigger unit is good if you have the dollars, but is not required. I have a Timney and like it and also have a Wilson that I need to install.
The AR 15 is as customizable as they come, so have fun with it. If you get to a point and want to change, say have a short barreled upper for one task and a longer barreled one for sniping / hunting, it is very easy to do. Just realize that the term "mil-spec" means "close enough" for tolerances and sometimes a little filing is needed to get the upper and lower pins to line up just right. Don't use a dremel tool, just a fine hand file will do. Changes to the buttstock are just as easy, depending on your usage.
