I was literally thrown in a room with a few manuals and 20 broken pins. The big clue is, like anything else, practice, practice, practice. Start with something easy, like Early Bally Solid State pins. Start with very simple stuff, like changing all the lightbulbs, or replacing all the rubbers. From there, you can move on to something a little more complicated, like rebuilding the flippers, or changing a pop bumper skirt. Get more than one machine, so you get an idea of what stays the same between different machines of the same brand and era. Once you have those down solid, most of what you will learn will carry over to other pinball machines. Every brand and era is different, but they also have a lot of similarities.
You will make mistakes. You will blow fuses. You will probably set something on fire. It's cool. It's part of learning.