First, you need an active Microsoft account to participate. If you do not have one, just visit the Microsoft account How-To page to learn more.
Once you have an active Microsoft account, you can redeem a Microsoft gift card to your account either through your Xbox console, Windows Phone, or online at https://commerce.microsoft.com. Once you redeem your gift card, the entire value of the card will be transferred to your Microsoft account. You can then use the money in your Microsoft account to purchase apps, games, videos, and other digital content at Windows Phone, Windows, Xbox Games, and Xbox Video stores.
Please note that the card or code may provide additional instructions on redemption, and may be subject to additional terms and conditions as provided with the card or code.
You can manage your account by visiting https://commerce.microsoft.com where you can view your balance, check when any part of your balance expires, or review your purchase history.
NOTE: You can only redeem gift cards to personal accounts. If you only have business or corporate accounts then you must create a personal account before you can redeem a gift card or code. Read more about account types.