Knowing CAD will be of value.
I would recommend that you start your learning by viewing the Mach 3 videos to get a flavor of how Mach3 is used.
Read the Mach Mill Manual.
Purchase CNC Machining Handbook by Peter Smid, read / study the first 100 pages or so, then continue reading it for
what applies to the Mill.
Then go back and re- read Mill Manual and pay attention to the gcode section.
Have a look at Sheet Cam for using the Plasma and the gcode generated..
Plasma machine operates differently than a mill or router and has it's quirks. You need an understanding of how the machine
operates.
The above will move you along the learning path.
As far as a plasma machine is concerned talk to someone who has knowledge of plasma cutting.
General CNC concepts apply to all CNC machines, but as said, each machine is different in use.
Enjoy the learning, CNC is not "plug and play", and there are no short cuts, so simply said:
Know before you go
RICH