Q: How to make your iphone use your itunes gift card instead of Credit Card

I recently purchased an itunes gift card because my "credit card" on my account was a gift card and i used all of the money on their. Every time i go to update an app or buy an app it tells me something went wrong with you credit card. I know what went wrong with it but i need it to use my itunes gift card instead. It also won't allow me to delete the credit card. Does someone know a solution or do i have to call apple support?