Redeeming iTunes card

This page has instructions for how to redeem the card on your computer or device (iPad, iPhone, iPod Touch), and it's value should then be added to your account : http://support.apple.com/kb/HT1574 . You should then be able to buy items from the store (apps, music, films etc) and the amount should be deducted from the gift card balance and not from your credit card.