1) Read every topic on this forum!
2) Visit all the industry websites to find out where you are going to buying your add-ons and toys.
3) Did I mention read every topic on this forum?
4) Start collecting your ammo and mags. You have to have those anyway so you can go shoot. Anything else is incidental to being able to shoot when you receive your AR.
5) Did I mention read every topic on this forum?
6) Figure out what sight(s) you want and plan ahead. You'll find that the two most common sights here are the EOTech and the Aimpoint. Advantages to both, go out and handle them. Get familiar. Read up on the others and check those out, also.
7) Did I mention read every topic on this forum?
8) Find a place to shoot. Some may limit FMJ or have certain restrictions that may limit your shooting.
9) See Note 1 above.