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 into Recovery Mode
1. Turn off iPad
2. Connect USB cable to computer; leave the other end alone
3. Press and hold the Home button down and connect the docking end of cable to iPad
4. Continue holding the Home button until you see the "Connect To iTune" screen
5. Release the Home button
6. Open iTune
7. You should see "iTunes has detected an iPad in recovery mode"
8. Use iTune to restore iPad
Note: You need to be patient and repeat the above many times to recover your iPad