Back up your data
Sign out of iCloud
Sign out of Messages
Unpair Bluetooth devices if needed
Restart your Mac
On Intel Mac, hold Command + R during startup
On Apple silicon Mac, press and hold the power button until Startup Options appears
Select Options, then click Continue
Enter your administrator password if prompted
Open Disk Utility
Select the startup disk
Click Erase
Choose the correct format if prompted
Confirm the erase
Quit Disk Utility
Select Reinstall macOS
Follow the on-screen instructions
Complete setup or shut down if selling the Mac