Normally a format will do, to stop someone accessindg data.
There are several data recovery programs that will recover erased files butstruggle after a format.
Pay some money and you can recover after a format.
Pay alot of money and you can recover anything.
If your paranoid then overwrite data with File erasers :-
Eraser click here
Ultrawipe click here
SuperShredder click here
killdisk click here