A cosigner on a credit card account guarantees that the issuer will have a secondary means by which to collect payment should the primary signer become delinquent on their account.  The most important thing to remember about a cosigner is that their credit scores and history are directly affected by the activity on the account that they’ve signed for.  However, because you are only seventeen you will not be able to be the primary signer on a credit card account.  Until you turn 18, your dad can add you as an authorized user on one of his existing credit card accounts.  Alternatively, he can open a new credit card account and add you as an authorized user on that account, with the agreement that you will be responsible for the bill.