I'd stock up on high capacity magazines while you still can first. Next, I'd get a light on it. Lots of options here depending on your budget. I installed a Surefire M500A on one of my M4s and I love it but there are several other good options available.
Depending on what you want the primary role to be, some sort of dot scope such as EOtech or Aimpoint would be my next suggestion. When it comes to the AR platform, you are only limited by imagination and money. Good luck.
Mags, a sling and a scope. Anything else, depends on what you are going to do with it. Defense? Hunting? Range only? Any combination of those? Of course, if you are a reloader, dies, bullets, powder, etc...
As for the many suggestions for a light... maybe this guy isn't going to need a light. Maybe he's only doing outdoors daytime or range only shooting.
Either way... welcome to the wonderful world of AR shooting! :-D