How do I use my iTunes card without validating a credit card?

I have an iPod and I had gotten an iTunes gift card and I have already set up the 15$ onto my account. Unfortunetly I do not use or have a working credit card. How can I use the money on my card without having to validate a pin code?|||You shouldn't have to..you already set up the $15 gift card, right? Whenever you try to make a purchase, the amount should be taken from the $15. Please message me more info.