Welcome to the wonderful world of the AR.
For ammo, I prefer Israel Military Industries (IMI) M193. US Milspec ammo manufactured in Israel. Boxer primed, reliable, and consistent, it is hard to beat. IMI M193s civilian production line twin is Winchester Q3131A. IMI is contracted by Winchester for the production of this ammo using the same materials and tooling as their M193.
Ammoman (Ammoman.com) has 500rds of Q3131A for sale at $109, including delivery. 21.8 cents per round.
Botach Tactical (Botach.com) sells 1,200 rds of IMI M193 for $189, including delivery. 15.8 cents per round.
Both Q3131A/IMI M193 are of recent manufacture (2001 or more recent). Ammoman has good (not great) prices and outstanding customer service. Botach is spotty when ordering anything other than ammo from them, however I've orderes ammo from Botach at least six times in the past couple of months and have yet to have a problem with an ammo order.
It would also be a good idea to get youself some mags. You can't go wrong with USGI mags. Ammoman is currently selling "gold standard" of pre-ban AR mags, new in wrapper Labelle teflon coated 30 rd mags (5 for $159). I already own ten (I'm stuck to purchasing pre-ban mags while I can, thanks to the permanent Pataki Ban in NY State), and they are the finest mags I have ever seen. For the range and just banging around, Ammoman is selling steel British SA80 30rd mags (5 for $79). Tough magazines that you won't care about dropping to the ground in a tactical reload drill, or just banging around in your bag.
You now have ammo, mags, the next must is a Lula. It is an ingenious Israeli invented plastic gadget that is used to load/unload AR magazines. It will save the feed lips on your mags and your own fingers from undue stress and damage.
Enjoy!