First of all shutdown your phone and then remove your battery for 5-10 seconds. After that put the battery back and press simultaneous the volume up button and the power button and hold then for about 15 seconds. Next use your volume up and down buttons to select "Wipe data/factory format", and use your power button as an "OK" button. After that select "Yes -- delete all data". Wait until the process is finished and reboot your phone.