You may also buy prepaid phones for a small price and then you must purchase minutes. Some of the most popular companies are Sprint, Cingular and Verizon. Expect to pay around $30-65 a month depending on your plan and how much time you spend on the phone.
The system used in America is standard GSM on 1900 which different from other countries so most cell phones from out of the country will not work within the United States.