This article explains how to redeem prepaid codes so you can make Xbox purchases and extend your subscriptions. It also explains how to check your Microsoft account balance.
Prepaid codes usually come in the form of a 25-character code printed on a card purchased from a retailer. You can use prepaid codes to make purchases without a credit card. You can redeem prepaid codes on your console or on a computer.
Note If you're on the Xbox 360 Entertainment For All Plan (US only), you cannot “stack” or combine prepaid subscriptions during the life of the contract. If you cancel the subscription before the end of the two years, you'll be charged an early termination fee and any applicable taxes.