will my credit card be charged if i have money on my i tunes card?

iTunes Store: How to redeem a code - http://support.apple.com/kb/HT1574 - If you have an iTunes Gift Card or other code that you want to redeem in the iTunes Store. Includes hierarchy of how things are charged.

Some items such as gift cards can only be purchase with a credit card.

Nope. As long as you have enough store credits, that will be consumed first then your credit card.

I have plenty f store credits but my credit card is being billed. I do not understand.