Really the best advice I can give you is to shoot your rifle for 200-500 rounds before you change anything. Try to make every round count don't just blast away. Get a feel for the trigger sights, handguards etc... It might just surprise you and you might find you don't need to change anything. Also the first upgrade I make on an AR15 is the trigger. It is the best money you can spend to improve your gun IMHO. A good trigger will tighten your groups more than anything. They are simple to install and require no special tools but again first get out there and shoot! So with that in mind.
First get some ammo. If you are looking for basic plinking ammo look at Wolf Gold, American Eagle X193, AE 223, PMC Bronze, & other brass cased 55gr ammo. See what your gun likes best. Ever rifle will be slightly different.
Sportsman's guide has Wolf Gold at about $.30 a pop for 260 rounds.
Wolf Gold, .223, FMJ, 55 Grain, 260 Rounds - 292054, .223 Remington Ammo at Sportsman's Guide
Palmetto state has a decent deal on X193 $6.99 a box buy 10 get free shipping.
Federal 5.56mm 55gr MCBT Ammunition 20rds - XM193F
If you want AE X193 on stripper clips with spoon Mass Ammo has 420 round cans for $159. Flat rate shipping $15.99
Federal 5.56x45 Nato/ .223 REM XM193 (420-Round Ammo Can)- FLAT RATE SHIPPING! - 4 max per order - MassAmmo.com - $12.99 Flat Rate Shipping on every order
Cabellas has PMC for $6.99 and$5 shipping with $99 purchase.
For self defense ammo look at Palmettos deal for 100 rounds of Gold Dots and 5 D&H mags for $79. This kills two birfs with one stone. The D&H mags are solid.
Five (5) Boxes Speer .223 55gr Gold Dot Ammo 20rds & Five (5) D&H 5.56 30rd Aluminum Mags
If you want other mags get Pmags, D&H, Brownells or other quality mags. Everyone has their own preference. I personally like Pmag gen2s these day because they are cheaper than Gen3 and I don't need the covers. I don't buy ones with windows. I pay about $8.50 shipped for Pmags. I buy when I can get that price. I suggest 10 mags for your one rifle.
From there as others have stated in order to put a forward grip on it you will need to change the handguard. You can go with a drop in like a MOE or Quadrail or you can swap out the A2 post and gas block and go with a free float tube. I would personal skip the foregrip. They are not needed. Learn to shoot the rifle with the stock guard. It is not that hard. After 1000 round or so if you still want it get a drop in.
Cleaning kits are a personal preference sort of thing. I like ones that have a tool to help clean the BCG. I like this Otis one because it all stores in a single pouch that you can throw in your range or go bag.
Amazon.com : Otis Modern Sporting Rifle & AR Cleaning System : Hunting Cleaning And Maintenance Products : Sports & Outdoors