Back up your files with Time Machine or another backup method
Sign out of iCloud
Sign out of iMessage
Sign out of FaceTime
Sign out of Music and TV apps if needed
Unpair Bluetooth devices if desired
Restart the Mac
On Apple silicon Mac, press and hold the power button until startup options appear
On Intel Mac, restart and immediately hold Command + R
Open Disk Utility
Select the internal drive
Erase the internal drive
Quit Disk Utility
Select Reinstall macOS
Follow the on-screen instructions
Set up the Mac as new or shut it down for the next user