You do not have to use a credit card to buy B&N ebooks. You can use gift cards. However, you must have a valid credit card/debit card with a credit card logo on your B&N account as the default credit card. Without one on your account you will not be able to buy ebooks at all. The card is used to create the encryption key to unlock the ebook on your NOOK. You will have to speak with your parents to make sure they are aware of this.