Apple imac computer. Override the existing password?

[Brian P]

Someone gave me an apple imac all in one the other day. It's an old one but still working. The problem is that it already has a password entered, and i don't know how to overcome this. When I click on the user icon it asks for a password and i don't have it. The guy I got it from says his son used it along time ago to play games on, and he doesn't remember the password either. Is there a way that I can override this so that I can log onto it myself? Any help would be appreciated! Thankyou, Brian