Accessing Mac mini's admin account after primary user's death

My dad, the administrator of my Mac desktop/Mac Mini, recently died, how do I access admin if I have nothing and I don't want to have to reset my entire Mac. When he passed he was the only one that knew the password for my mac (the mac is mine cause he gave me it for my birthday but he had absolute control of my downloads and stuff and im 18 now). It's not just downloads i can't access, it's being able to print things, update my computer, and even deleting apps. I can't do anything about it unfortunately. please help

[Edited by Moderator]