how to reset admin password on macbook air 10.8.4 OS X mountain lion or make a new admin account? (I think the mavericks disk or w.e is locked)

So this is the first mac I've ever used so bear with me but pretty much my dad gave me his 13" macbook air (mid 2013 version) because I'm ravelling overseas and needed a laptop. Anyways I can't log into the admin account because I don't have the password and I can't get ahold of my dad so I've been using the guest account. I've tried everything including this OS X Lion 10.8.4 - Admin Password but when I hit "resetpassword" the window which comes up contains nothing in the user account box. I have also tried the "create new adminstratative account" option but when I type "rm /var/db/.AppleSetupDone" in command nothing happens. Also the mavericks disk is locked. I've literally tried everything I could and still cant use this. Does anyone know what I can do?