Reddit reviews macOS Support Essentials 10.12 - Apple Pro Training Series: Supporting and Troubleshooting macOS Sierra

ex Apple Genius here... Hope some of this helps.

• Get this book for a start: https://www.amazon.com/macOS-Support-Essentials-10-12-Troubleshooting/dp/0134713850/ref=sr_1_1?s=books&ie=UTF8&qid=1497477151&sr=1-1&keywords=macos+support+essentials+10.12
  
  
• Make a diagnostic drive. Get work to buy you a few portable external hard drives. (I like LaCie) Install the latest 3 versions of macOS on their own partitions (~32GB each) and leave the rest to a "Free Space" partition. When troubleshooting a Mac, the first thing you should do is boot the machine from one of the "known good" OSes on your diagnostic drive and check if the issue persists. That tells you if the issue is hardware or software.
  
  
• If you've determined the issue is software, you can usually divide it into a System issue or a User issue. Create a new user on the system and check if the issue persists on the new user. If so, it's a system issue. If the problem goes away, it's a user issue. System-wide settings generally live in /Library/Preferences/. User settings live here (mostly): /Users/username/Library/.
  
  
• Make an installer drive. Use createinstallmedia to make an external HD full of all the different OS installers you need. Learn how to NetInstall macOS if you need to deploy on a lot of machines.
  
  
• Malicious software usually has to be installed as a Launch Agent or Launch Daemon in order to run when a Mac boots. Learn how they work, how to find them and how to destroy them. https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html. Also learn about LoginItems & StartupItems.
  
  
• Learn the boot process of a Mac. https://support.apple.com/en-us/HT204156 and the startup modes https://support.apple.com/en-us/HT201255
  
  
• Make sure every computer you touch is backed up!
  
  
• Learn the Terminal. (bash) if you don't know it already.
  
  
• Get a developer account for early access to beta software.
  
  
• Get decent tools (Wiha) and anti-static gear.
  
  
• Take hardware problems to the Genius Bar, especially if your company is gonna pay for it. They'll usually have better diagnostics than the Mac has built in. It also rules out the possibility of you breaking a computer when trying to fix it. (edit: also a great excuse to get out of the office)
  
  
• Use Google. Chances are someone's has had your problem before. Google is also a great way to search support.apple.com
  
  
• Be excellent to each other.
  
  
• Party on dudes.

https://www.amazon.com/macOS-Support-Essentials-10-12-Troubleshooting/dp/0134713850

Read it and understand it.

Also a good source is http://macadmins.org