Hey AndrewTarr,
It sounds like you're wanting to know how to use your Apple ID balance instead of a credit card when making purchases in iTunes and the App Store. The following resources have additional information on adding funds, how billing works, and redeeming gift cards:
How App Store and iTunes Store purchases are billed
Create or use an Apple ID without a payment method
What type of gift card do I have?
Check your Apple ID balance
We hope this information helps. Thanks for using Apple Support Communities.
Have a great day!