Your Name (top-right of screen) > Account Settings > Payment Method
To save your credit/debit card information for future purchases, click on your name at the top right of the screen and click on "Account Settings". Under the "Accounts and Settings" section, click on the "Payment Method" link. Enter the appropriate information in the fields of the form and press the "Submit" button to save this information.
You can also save your card information, while paying for a class/class by checking the checkbox 'Save card for future payments'.