If the requirement is to erase all the data from the hard disk drive on the Mac, then it's best to use a tool known as the Disk Utility. This will delete all the files and volumes on the hard drive. The Disk Utility offers four options:
1. Don't erase data2, Zero out Data3. 7-Pass Erase4. 35-Pass Erase
On the other hand, if the requirement is to delete individual files, then the best thing to do is to highlight the file you want to delete, right-click on it and "Move to trash".