You walk into an At&t store, ask to get a plan and that you already have a phone, and just need a SIM card for it.
You can get a contract plan or a month to month without contract, pay as you go prepaid or whatever you want. As long as the phone works, it should not be a problem getting a sim a new number and all set up before you leave the store.
I would go with a pay as you go for now, at least until you find out how well the signal is at home, and places you spend the most time.
