Unlocked Iphone 4s off apple website?

I'm thinking about purchasing a unlocked iphone 4s on the Apple website. I want to use the At&t carrier. So do i just get a at&t sim card? And a pre paid card to use the Iphone? Or do i have to buy a nano card? What exactly is a nano card or do i not need it? Can you please tell me what exactly i need to get. Thanks.

Added (1). I don't want to get a contract plan.

If you don't want a contract plan, do go for AT&T. Buy a iPhone 4S factory unlocked and get a prepaid SIM from StraightTalk and you're all set. You'll need a microSIM for the iPhone 4S, nanoSIM is for iPhone 5. You can mail me if you want to buy unlocked iPhone at very reasonable price.