Mike has written a novel, but he has to send in his computer for repair and is worried that even if he deletes it, it'll be recoverable. How can he be sure? Leo says to first make sure he's made at least three copies of it so it's backed up. Then he can erase the computer by using Apple's built-in "secure erase" feature. He'll have to reboot the Mac and hold down Command and R keys, and then launch Disc Utility. There's a secure erase feature in there that will write over the drive several times and remove all the data. Nobody will be able to recover that.
If Mike doesn't want to erase the whole drive, he can also securely erase just that file. To do that, he can drag it into the trash, and then in the Finder menu, he can select "secure empty trash." That will do the same thing.
For Windows, DBAN (Derik's Boot and Nuke) will securely erase the drive. Always back up the computer before bringing it in for repair because they will either replace the computer or they may wipe the drive during the repair to get it back to it's original configuration.