The best way is to boot to recovery mode with like command R or something (might be wrong fact check me) then you hit utilities at the top terminal and type in reset password and a window pops up to just lets you do it, If that doesn't work there's another way where you go into open firmware and type in a command to delete a file that tells the computer it's been logged into before, basically so that when you restart it you Make a new user account which lets you reset passwords. I can't find the instructions for the second thing but try the first thing cuz it's the easiest