1.Turn off your phone
2.Press together volume up + home button + power button
3.Release your fingers of the phone when Samsung logo appear
4.Choose with volume buttons "wipe data/factory reset" and select it with power button
5.Choose "yes" with volume buttons and select it with power button
6.Wait untiil format is complete
7.Select "reboot"
8.Wait until the phone restarts
That's it!