First off, you'll need to be logged in to redeem a card to your account.  Treat gift cards like cash - they can only be redeemed once and once they are redeemed they're locked to the account they are in.

To redeem your gift card go to "My Account Information" by clicking on the top right menu and clicking on Account Info.  

On the My Account Information page click on the button for "Redeem a Gift Certificate" at the bottom of the page under "Store Credits & Gift Cards"

Enter the gift card code in the next page and you'll see a confirmation that the gift card value was added to your account.