For routine uses, I recommend “Darik’s Boot and Nuke” (DBAN) using the “DoD Short” method.
But, if you have any personal information that you feel very concerned about then use the DBAN “DOD 5220.22-M (7 passes)” method.
Or, if possible, just remove the drive from your computer and smash it thoroughly with a regular hammer making sure to warp the disks and crush the circuit board.
If you have access to an industrial shredder, you can just toss the hard disk drive into it.
If the computer is going in for repair, make sure you have whole disk encryption turned on. That way, as long as your user account isn’t compromised, no one will be able to read any of the data on your disk.