If you own an iPod touch, you have the option to set a password, which Apple calls a passcode, to protect your data. Anyone who attempts to access your iPod, after unlocking it or turning it on, will require the passcode to do so. However, if you forget your passcode or incorrectly enter it, you will eventually see an alert that your iPod touch has become disabled. To unlock your iPod touch after forgetting the passcode, and still access your content, you must connect it to the computer with which you last synced it.
Connect your iPod touch to the computer with which you last synced the music player. Launch iTunes if it does not automatically open. Select your iPod touch under the devices list, and click the "Summary" tab in iTunes. Click "Restore" and, when prompted, click the "Back Up" button to save a back up of your iPod touch in iTunes. Click "Restore" when prompted, after the back up saves. After restoring, click "Ok." Choose the most recent back up from the drop down list on the "Set Up Your iPod" screen. Click the "Continue" button to restore your iPod touch.
iOS: Forgotten passcode or device disabled after entering wrong passcode http://support.apple.com/kb/ht1212
iTunes: Restoring iOS software http://support.apple.com/kb/HT1414