how can I clean my HDD

I am sending my computer out for repairs, so I've been thinking along the lines of what would happen if I needed my hard drive cleaned. Now I realize there's not really any risk here at all, but now my mind's focused on what I'd do if I'd need to erase all the bad shit on my hard drive and leave no evidence, aside from just breaking it or something like that. Just tossing thing in the recycle bin and deleting them doesn't make them go away, I understand! Also shit in my web cache etc