If you have access to the Internet, tap the "Forgot Password?" option that appears on your screen, enter the Google account that is used with the phone and use the password received via email to unlock the device. If you can't connect to the Internet, you will need to do a factory reset of the device. Turn it off, press and hold Volume Up + Home buttons and then press Power. Select Wipe data/Factory reset from the Recovery menu using the volume keys and follow the on-screen instructions.