how to fix disable on my ipad?

If your mean that your iPad is showing the red disabled screen due to too many incorrect passcode attempts to unlock it then you will need to connect the iPad to the computer that you normally sync to and you should then be able to reset the iPad and restore/re-sync your content to it (http://support.apple.com/kb/HT1212) - you may need to put the iPad into recovery mode in order to be able to reset it : http://support.apple.com/kb/ht1808

If you do it via a different computer then :

If you restore on a different computer that was never synced with the device, you will be able to unlock the device for use and remove the passcode, but your data will not be present.

If you mean that your iTunes account is disabled then you might be able to re-enable it via this page : http://appleid.apple.com, then 'reset your password'

Or you might need to contact Apple : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page

If it then works on your computer's iTunes but not your iPad then try logging out of your account on the iPad by tapping on your id in Settings > iTunes & App Stores and then log back in and see if that 'refreshes' the account on it