I am using AT&T. I am an international student and I was told that my cell phone would not work with US sim cards. However, it works so I did not have to buy a new one - which is awesome. There is a chance that your iPhone with work with ATT's sim. I was told, actually, that iPhones usually do work with American sims, no matter where they were bought. I give no guarantee but there is hope If it does not work though you will have to buy a local cell phone. And most probably you will not be able to use a contract because you have no credit history in the US so you would have to deal with deposit. And deposits are huge here. ATT wants 500 bucks! If you want to use your iPhone with American sim card, Verizon is out of the question because they have no sim cards at all. So go to an ATT office and ask them to try putting their sim card in your iPhone. If it works - you are lucky Then you can use their unlimited prepaid plan (no contact) which is 50 bucks a month for unlimited talking, texting and data (internet), which is not a bad deal compared with other plans that are out there.
If you have more questions, feel free to ask