Connect the iPad with the USB cable to your computer.
If not automatically, open iTunes on the computer.
When prompted for a password, which you do not, remove the USB plug from the iPad (not the computer).
Turn the iPad off (sleep button pressed for a few seconds until the red slider appears, and then turn off the iPad).
Connect the iPad again with the USB cable still inserted in the computer while holding down the Home button. Let the home button until release when you see the message "Connect to iTunes" or "Connect to iTunes".
iTunes detects that an iPad is connected in recovery mode (English: recovery mode). Click "OK" and select "Restore" or "Restore" to restore the iPad to factory settings.