Credit card removal
How do I get my credit card off of the iTunes so my son can't use it without me knowing
iPod touch
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 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 : http://support.apple.com/kb/HT1918