I have spent most of two decades contracting to businesses that needed temporary system administrative talent, with a few 'permanent' jobs here and there. Most of these jobs became available because the previous system administrator left the business suddenly. I rarely had a chance to start a position while the previous person was still available to show me what they had done and explain why they had done this or that a certain way. I adopted "Don't panic" as my main rule at work. ( From "The Hitchhiker's Guide to the Galaxy" ). This rule has proven its worth many times. You want the people that hired you to feel comfortable that their systems are in good hands. Even when you feel that the sky is falling it is important to convey a sense of calm to your manager and end users.
My next rule is "Don't make changes unless you understand why things are the way that they are". What I mean by that is that you have to figure out why the previous system/network administrator made configuration choices before you change the configuration. You may see something, like file access policies, that don't seem to make sense. You have to understand why the configuration was set up the way it is before you change it. Otherwise you can break something. For example, if everyone on the LAN is given full access to the files of the Accounting Department then you have to figure out why this is set up this way. Maybe it is just that the previous person didn't know what they were doing, or maybe there is an application that is run by people in another department that reads and writes to the Accounting Department files. Make sure you understand what is going on before you make changes.
My last rule is simply to be friendly and courteous to everyone. Respect people. I have found that if people like you then they will be patient with unexpected problems that you create.
Those three rules will serve you well when starting a new job. Your confidence will give people the idea that you know what you are doing. Your understanding of the systems before making changes will reduce the number of problems that you create. Your systems will be reliable. People will love you.
