wiping an Apple HD clean

Need to clean off an Apple iMac before it is sold:


Start up from macOS Recovery
macOS Recovery installs different versions of macOS depending on the key combination you use while starting up. Turn on or restart your Mac, then immediately hold down    Command (⌘)-R

To erase your startup disk:

1. Select Disk Utility from the utilities window of macOS Recovery, then click Continue.
2. Select your device or volume in the sidebar of Disk Utility.
   
3. Click the Erase button or tab.
   
4. Complete these fields:
   
   • Name: Enter a name for your disk, such as "Macintosh HD".
   • Format: Choose Mac OS Extended (Journaled) or APFS.
   • Scheme (if available): Choose GUID Partition Map.
5. Click Erase to begin erasing your disk.

https://support.apple.com/en-us/HT204904