After you register for your iTunes account for your iPhone or other iOS device, you might decide that you no longer want to keep your credit card information in your account. Fortunately, it’s easy to remove that information. All you have to do it log in to your account, choose the Store menu and then choose “View my Apple ID”.
When you see your Apple ID Summary in the “View my Apple ID” in the iTunes store, click on Edit to the right of the Payment Type, and then just click on None. Make sure you also click on Done at the bottom of the screen so your new option to not have a credit card will be saved.