A good way to get started is to get a copy of the Apple Service manuals for all the older Macintosh computers. You can often find them sold as PDF files on CD or DVD, generally on eBay for very cheap, which in my opinion is the best way to have them. They are also available in certain places online that host these manuals but this is generally not as consistant as a disk version. These manuals are written for practically each and every model, though there are some that apply to a line of similar machines. They describe the computers in great detail and always have a "take apart" section as well as a section devoted to instructions for repairs. Work through the one for your model step by step when fixing a problem and you will be amazed at what you can learn. If you can, find a broken machine and use it as your "practice Mac" for taking apart and practicing repairs - then it won't matter if you make a mistake. Start small - get used to the basics such as removing case plastics, internal plug connections, hard drives, RAM etc first before moving onto other things such as tinkering with video circuitry. Spend some time over it and be patient - practice makes perfect.

This is how I learned about taking apart and repairing old Macs and although many tasks have become second nature, I often still find myself referring to the manuals when carrying out an unfamiliar task.