I AM SO MAD! WHERE CAN I BUY A LOCK FOR MY DOORKNOB?

MY SISTER KEEPS ON TAKING MY STUFF! I HATE HER! HOW DO I LOCK MY ROOM EVEN THOUGH THE DOORKNOB DOESN'T HAVE A LOCK?!? DO I BUY A DOORKNOB WITH A LOCK, TAKE OUT MY OLD DOORKNOB THEN INSTALL IT?!?