forgot log in password on another mac

Have a macbook pro about four years old and have stripped it to make it available to another user. tried to open it up and was confronted with a need for a password which I don't believe I had set but did try the password I had used before I stripped it and this didn't work. Nor dod others I might have used. tried directions for resetting provided by mac support and nothing works. How do I get this thing opened? Whitey, St. Paul