You'll want to "write" over all your data. You cannot completely destroy data only write over it. Even then there's a thing called slack space that can still leave residual data. But if you are just trying to clean up everything so a normal user can't access it and you're not avoiding law enforcement then you'll be fine with below.

http://www.geeksquad.com/do-it-yourself/2mm/hard-drive-wipe.aspx