If you cannot remember the passcode for your iPad, iPod Touch, or iPhone, you will need to restore it using the computer with which it was last synced. This allows you to reset your passcode and resync the data from the computer, and restore the device 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.
Refer to Restoring iOS software.
To restore from a backup, read How to Back Up.