I see you'd like to use an iTunes Gift Card in replacement of the payment method you have for purchases using your Apple ID. I'm happy to help.
Change or remove your Apple ID payment information -- This article shows you the different places where you can change/remove your payment information.
Payment methods that you can use in the iTunes Store, App Store, and iBooks Store -- This article informs you what types of payment methods are accepted.
You can typically use these methods:
- Credit or debit cards
- Store credit, from redeeming App Store & iTunes Gift Cards or Apple Music Gift Cards
Redeem App Store & iTunes Gift Cards, Apple Music Gift Cards, and content codes -- Follow the steps in this article to redeem your gift cards and codes.