You've redeemed the card onto your iTunes account e.g. via the Redeem button at the bottom of the Music tab in the iTunes store app
Redeeming a card : http://support.apple.com/kb/HT1574
If you have then you can buy songs by tapping on their price buttons in the store, as long as your account's balance covers the purchase then only that will be charged - if it doesn't cover it then any remainder will be charged to the credit card on your account