Q: i have iTunes credit and would like to use it to pay for a purchase, How?

If you haven't yet redeemed the card or code to add it to your iTunes account, the following has instructions: Redeem iTunes Gift Cards and codes - Apple Support

Once you have done that, the balance will show on your account and it will automatically be used first (before your credit card or other regular payment method).

Also, this being December 24, we should add that a credit balance from gift cards cannot be used for the purchase of gift items.  If you are purchasing a gift, iTunes will use your credit card, even if you have store credit.