switch off your mobile. Press the volume down and power button simultaneously for 10 seconds.
An android logo will appear and disappear automatically and the phone will show the recovery menu.
By using volume key go to option “Clear Storage” and confirm it by using power button from the menu having options like Recovery, Sim-lock, Clear Storage etc. This will start the resetting process and after the process is over the phone will restart will factory settings.
you will lose all of your sms and apps,so you must back up your apps and sms.