Q: How do i remove my credit card information from my iPad

Seehttp://support.apple.com/kb/HT1918 for instructions.

If you have a credit card on your account then it has to be valid, if it's expired then you will get an error message when trying to download anything.

To remove your card's details from your account then on your iPad tap on your id in Settings > iTunes & App Store and then tap on 'View Apple ID' on the popup and log into your account  - that should give you a payments link on your account's page. Or you can remove it via your computer's iTunes by going into the Store > View Account menu option and logging into your account, and on your account's details page there should also be a payment link.

Changing payment info : http://support.apple.com/kb/HT1918

If you don't get the 'none' option on your account's payment details screen : http://support.apple.com/kb/TS5366