The Internet is full of all sorts of good information about buying, collecting, repairing, and restoring pinball machines. This is my short list of good references. If you are new to pinball machines, these will provide plenty of information to get you started, and there is much more you can search for.
Pinball machines can provide enjoyment in two major ways. Some people prefer one, some prefer the other. I am a fan of both.
Before opening any pinball machine on your own, be sure that you are following good safety practices to prevent injury. The Pinball Safety page has some general guidelines.
(BOOK) Pinball Machine Care and Maintenance. If you are just starting out, this is a good starting point to learn about the different pinball machines, and how to buy one and repair it. I started out with this book and found it to be an enjoyable way to get familiar with pinball machines.
The Internet Pinball Database. If want to know about a specific machine, you can look it up here and learn all kinds of useful information about it.
Pinrepair.com. An almost unbelievable repository of information about how to work on and restore pinball machines. If you read every single on here and understand it you will be well on your way to becoming familiar with how pinball machines work.
Pinside Forums. In addition to having different forums where all sorts of things about pinball machines are discussed, including repair and buying advice, there are many excellent guides to help with all aspects of repairing and restoring old and new pinball machines.