Instructions
Log in to your Xbox Live profile.
View the "My Xbox" area of the dashboard menu.
Scroll right and choose your Gamertag. New options appear.
Scroll to the Account Management area.
Choose "Manage Account." The Your Billing Options page opens.
Open "Manage Payment Options."
Choose "Add Credit Card." This option also applies to debit cards.
Enter your credit card information into the corresponding fields, select "Next" and confirm the details on the following prompt.