You can choose a GSM carrier (Cingular or T-mobile). You will need to purchase a SIM card from whichever carrier you choose (approx $20). You might have to sign at least a 12 month contract (depends on the carrier).
I just bought an unlocked Nokia 6620 phone off of ebay. What kind of service plan options are available (I don't have a service plan now and I don't have a sim card)?