Inherrited Macbook but can't login

My uncle who was a computer geek recently passed away and as his only living relative left, I inherrited his estate and amoung his belogings , inherrited his macbook. I have no idea what model or serial number and was wondering how or what are my options to gain access to his old mac? Is there a secret manufacturer code I can initiate at boot up or thirdf party software I can use on a usb stick I could use? I don't even have the OS software to try and reload. Am I just out of luck or is there something I can do? and how much? Thanks in advance for your response.