How do I take credit card off and just use ITunes cards for Ipod
HHow do I redeem ITunes cedit and not use my credit card
iPad Mini, iOS 7.1
On your computer's iTunes you should be able to edit your payment info by going into the Store > View Account menu option and logging into your account, and on your account's details page there should be a payment link. If you are doing it on your iPad or iPod then tap on your id in Settings > iTunes & App Store and tap on 'View Apple ID' on the popup and log into your account - that should also give you a payments link on your account's page.
Changing payment info : Change or remove your payment information from your iTunes Store account (Apple ID) - Apple Support
If you don't get the 'none' option on the payment details screen : Why can’t I select None when I edit my Apple ID payment information? - Apple Support
To redeem a gift card onto your account : Redeem and use iTunes Gift Cards and content codes - Apple Support
As long as you are buying content for yourself (you can't use your account's balance for gifting) then your balance should automatically be used for the purchase