I'm going to assume that you purchased your iPhone in Europe and that it will not function on Verizon or Sprint because they are CDMA.
With that assumption in mind, I would recommend walking into a cricket wireless store and buying whatever plan that fits the amount of data you expect to use while in the states.
Cricket Wireless is owned by AT&T and works on AT&T's network. They are AT&T's "budget" prepaid plan.
If you wanted to, you could go to a Web site like NumberBarn and buy a phone number now so you can let people know in advance what number you will have when you get back to the U.S. Then you could port that number from NumberBarn over to cricket when you actually arrive.
If felt like holding onto that number, you could port the number back to NumberBarn when you left and save it for the next time you travel back to the U.S.