Contract is up; How do I get out of a family plan

You would do a transfer of billing responsibility. This would move that single line to a new acct with your name using your credit information.

To move your line from one account into another, you will need to perform a "Transfer of Billing Responsibility." To complete this, you will need the account holder (or an authorized user) on both accounts to work with AT&T to perform the transfer. This is not a complex process, but there are some conditions which would prevent the transfer. You should be able to complete the entire process over the phone, but you may need to go into an AT&T corporate store so they can verify the ID of the people involved.