First of all, there are 4 major carriers in the U.S. Cingular, Verizon, T-mobile and Sprint. Then there are some smaller carriers and some companies that resell time on the major carriers.
Determine where you are going to go and check the coverage maps. If you leave an area covered by your carrier you are roaming. Check to see what the carrier has to say about prepaid roaming. Some will not allow it, others may charge a premium.
And finally, consider how many minutes a month you are going to talk. If you you are going to talk quite a bit, or go outside of a carrier's coverage, you may want to consider a monthly plan. Cheaper and usually you can roam free of charge, or at least not as costly as the prepaid plans.
Finally, the phone. Get one that you are comfortable using, that you can understand how to use, and one that is easy for you to carry.