Ask HN: Tips on Sysadmin Job

Recently I got a promotion at my job. I was working with another guy in a small company with an even smaller IT department. That guy was fired and I got the position to be the only sysadmin at the moment. I never had any experience working and managing the whole IT infrastructure, because the company has 4 campuses around the city. I'm kinda confused on how to manage everything, my background is with information security and it's going to be a big challenge for me because it's my first job.