I just bought a refurbished MacBook and can’t log in

What you'd have to do is restart holding down the Option key. This gets you the startup manager. Pick the disk with the Recovery partition on it. hold down command (apple+R) after picking it and restarting. Move your mouse to the top center of the screen until you see OS X Tools. Go to Terminal. Open that up. Type in "resetpassword" without the quote marks. press the return/enter key on your keyboard. This should get you to the admin. (main) password reset app.

john b