Right now, more ammo if you can luck up on it without breaking the bank. Seriously shoot it a whole bunch until it's familiar as an old shoe, and decide from that what kind of shooting you'll probably want to do with it; e.g., home defense, long range work, tactical games, varmint hunting - then you'll know what you might change or add to your satisfaction. There's so much stuff out there for AR's, you can easily waste a lot of money trying stuff and end up not liking it or needing it.
In the meantime - scrounge ammo, magazines, and read.