In order to get past a forgotten PIN, you will have to reset the phone. To do this, press and hold the volume down and home while powering on the phone. When the recovery menu appears, use the volume buttons to select the wipe/reset option and use the power button to select. It will take a while to reset, but will eventually come up to the setup. Go through the setup using the same gmail account you used before so that the data will synchronize.