I have said this before and will say it again (and again), the accessory you NEED is AMMO!!!! Any brass cased US made ammo will work. No accessory will enhance your rifle like ammo. A case of ammo down range will give you a good idea of the shortcomings of your rifle.
Adding doo-dads to a new rifle is like buying a new car, changing out the rims and stereo, adding a turbo charger, nitrous, ground effects, window tint, seats, etc before you even have 200 miles on the odometer.
Sling? There are many very good options including Tapco and Blueforcegear