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.