If you have a balance on your account and you are buying content for yourself (you can't use a balance for gifting) then your balance should be automatically used.
How purchases are billed : http://support.apple.com/kb/HT5582
When making purchases, content credits are used first, followed by Gift Certificate, iTunes Card, or Allowance Account credits; your credit card or PayPal account is then charged for any remaining balance.
If you want to remove your credit card details on your account then follow the instructions on this page for changing payment information : Changing payment info : http://support.apple.com/kb/HT1918 - on your device you should be able to do it by tapping on your id in Settings > iTunes & App Store and then tapping 'view Apple id' on the popup and logging into your account