How do i return a cell phone to AT&T?

I bought a cellphone that i am NOT happy with and i want to return it. It is not even 30 days old so i can return it. but the problem is i dont think i have the right receipt. i paid with a credit card. my receipt just says ''prepaid customer'' and ''account balance'' stuff so is this the right one? also if it isnt shouldnt my purchase be on record on one of their computers in the store i bought it from. Or did i just not get a receipt with it because i paid with a credit card? Please help me with this because im really stressed out. that was the only receipt i got, i think. but i really need to return this and get a new one because i simply cannot deal with this phone! thanks