If you start your career with mainframe, its always hard to get at good understanding of what you will face in the future and how to start. Please find this "Roadmap" as a help for you. Any comments are highly welcome....

Seeing this Roadmap, I suggest that everybody should know about the Mainframe Hardware, z/OS and using ISPF, as well as Submitting Jobs using JCL.
But then the Roadmap will differ, depending on which career path you choose.
E. g. if you are going to be an application programmer (using traditional mainframe languages, or common languages like Java), you will not be confronted with the tasks the operators will have to face.

So, knowing everything for everybody is impossible. Try to get a good overview, and then concentrate on your tasks ahead.
This Roadmap is not a reference of "all" material regarding the sepeate topics, but its a start.

Good luck!