If you cannot remember the passcode, you will need to restore your device using the computer with which you last synced it. This allows you to reset your passcode and resync the data from the device (or restore from a backup).
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.
You may have to force iPad/iPod into Recovery Mode
If it's disabled because the passcode was entered incorrectly too many times, back it up and restore it on the last computer it was synced with. See "If you have previously synced your device with iTunes" here: http://support.apple.com/kb/HT1212.