I have a question about signing up for the iPhone.

You would need to go to ATT and have them run a credit check to see. It could be nothing. It could be $250. There's no way to know.

If you don't pass at&t's credit check for a post paid account (at&t will say no one fails their credit check - but subscribers that don't meet certain criteria credit wise must put down a deposit), a deposit will be required. If you have good payment history (paying on time and in full) for 12 months, the deposit will be refunded after 12 months.

You need to contact at&t to determine the deposit amount if required. If at&t requires a deposit, and you agree to the amount requested, at&t should give you a credit approval number - after the deposit amount is received. I'm not sure if they accept a credit card for a deposit, but likely.

AT&T is not that strict on their credit check, they do however pay closer attention to if you owe other wireless carriers and utilities money. Not having credit is usually not a issue with wireless providers. You can call them and they will do a pre-credit check. This will give you a code on exactly what they will charge you in advance before buying a phone or signing up with them.