It really depends on what you plan to do. Are you merely designing prototypes, actually making them, or both? And what sort of prototypes are you hoping to make?
I am a CNC machinist, and often work on “First article” parts; our term for prototypes. While I do not design them, I have to make them, and that involves a lot of feedback as the designer and the maker hammer out the best, most cost-effective (and time is money) way to turn your idea into an actual object. For a 3D printer, it’s easy. For a CNC mill/lathe, it’s more involved, but also variable depending on teh machine, the material….
Without knowing more details, it’s hard to say what you really need to know. The only constant is that CAD skills are required, though how much CAD you need to know depends on which end of the prototyping process you are on.