Set a firmware password
Most modern Macs support firmware passwords, but if you’re unsure yours does visit apple.co/2qZOzL3 to check whether your model is compatible. Restart your Mac and hold ç+r to open macOS Recovery. Once that loads, click Continue on the language selection screen if it appears.
From the menu bar, choose Utilities > Firmware Password Utility. In the new window that opens, click Turn On Firmware Password. You’ll be prompted to enter your password twice. Make sure this is different to your user account’s password, and note it down in a safe place. Click Set Password when you’re ready.
Restart the Mac. You will be able to log in as normal. However, from now on if you or anyone else wants to start up from a different drive, enter single-user mode, or access macOS Recovery, the firmware password will need to be entered.