The main thing is understanding how it all breaks down. There are not many mechanical parts. If you've worked on a computer, all of the same stuff is there: system board, controls, display, power supply, etc. As a beginner, you probably will not want to mess with monitor or pcb repair. Many problems are easily fixed, anyway. Control problems are usually loose wire or a bad switch, for example. In general the parts on these games are interchangeable. A joystick connects the same as a joystick. There are really only 5ish types of monitor, and one is more common than any of the others.
There's no need to be afraid. We all begin somewhere. The collecting community is so vast. Use Google for specific problems, chances are good that somebody has run into the same issue with that game or a similar game, or a game that uses the same type of monitor, etc.
It also helps to lurk around repair forums. The forums over at KLOV are a great source of info and help.