If I just buy an AT&T iPhone 5 can I just put my sim card in it and it'll work?

I wanted to buy a new iPhone, and didn't want to have to get a new contract. I wanted to know if I just bought a new iPhone off the internet with my company would I be able to do this, because I'm really getting sick of this iPhone 4. Someone please help me with this question.

Yes you can, as long as it's not lock to a service provider. You need to purchase a plan and get a sims from the provider.