Which cell phone carrier should i choose?

i need to find a cell phone carrier that will work in my area, but the problem is most of them dont work. any suggestions on a carrier that works out in the countryside? i know for sure that AT&T works, but thats about it. one salesperson told me that the carrier doesnt matte, its the phone you get. so if you have any suggestions on a phone, carrier, and a store where i can get the phone please please help me. thx, -Skittles