How you do this depends on which Mac you have.
What do you have?
If it's an m-series Mac, you can use the "Erase all content and settings" option (in system settings) to return the Mac to "moment zero" (the moment you first powered it up after taking it out of the box).
If it's an Intel-based Mac from 2011 or later, you can boot to internet recovery, then use Disk Utility to erase the internal drive, and then use the OS installer to install a fresh copy of the OS onto the drive. This obtains "moment zero" on an Intel Mac.
ONE OTHER THING: Before you do Internet Recovery, SIGN OUT of iCloud, iTunes, the App Store, etc.
If it's so old it can't do internet recovery, then boot from an external drive and use disk utility to erase the internal drive. The "one other thing" applies here, too.
