you have to boot into recovery and wipe data or factory reset..
remove the battery and wait for some seconds then insert the battery again
hold volume down and power button together until u see a recovery menu, then choose wipe data/factory reset us volume down/up to navigate .. then restart when done..
WARNING!personal data Like SMS/MMS/Contacts/Apps/call history .... will be gone for good if you need those then consider taking the phone a pro for backup..
Media such us pictures/videos/musics will not be affected