Depends on what you want to do. Hands on always works best for me. If you have JAMF in your organization see about getting access to the sand box, and learning over there. Pick a task, like build a package to deploy it (you would need a policy, scope, trigger, ext) and try to make it happen.
Going over JAMF 200's course material would give a good guideline for what you want to learn next. Also don't forget JAMF has a training portal if you are on your companies JAMF account.
Jamf 200 Course - Course Resources | Jamf
If you don't have access to a corporate JAMF instance. Look in to getting a JAMF Now account, its free. Upgrade it to the JAMF Now Fundamentals which is also free and comes with JAMF Composer and access to JAMF Support. JAMF Now really sucks as a MDM, but it is a MDM and you can play with it and it comes with JAMF Composer. JAMF Composer is what JAMF Pro also uses to build packages, you can use other tools to build packages but JAMF Composer is really nice at what it does, and its free with a GUI.
As far as Mac administration itself. Start with CLI file manipulation, which is all covered in JAMF 200. I can give some commands if you need them.
