How do I Remove personal data from iMac running MacOS High Sierra 10.13.6?

See: What to do before you sell, give away, or trade in your Mac - Apple Support

Create a second admin user, login in as that user, remove the other user electing to remove the Home folder as well.

All of the first user's data will be gone.

For the Apple ID and its associated data, sign out and don't use that ID any longer.