You cannot reset a passcode. You have to restore the device within iTunes. You want to use the same computer that you always sync with so that you can restore your app data and settings. You can restore with any other computer, but you will lose everything on the iPad. You will probably need to use recovery mode.
iOS: Wrong passcode results in red disabled screen
This will tell you how to restore
iTunes: Backing up, updating or restoring iOS software
If you cannot restore that way, you may have to use recovery mode
iPad: Unable to update or restore