Car mechanics learn their trade a number of ways. They do not all have the same background or education, and their training experiences vary widely. There is no one path to becoming a mechanic, but there are common elements of the paths that can lead to a career in this field. A combination of formal and informal training may work best for you.
Work in a garage. You may start out changing oil and tires or running parts, but you will learn a lot by just being around cars. According to the Occupational Outlook Handbook, 2010-2011 Edition, published by the Bureau of Labor Statistics, "Those new to automotive service" will often begin as trainee technicians, technicians' helpers or lubrication workers. "In many cases, on-the-job training may be a part of a formal education program." Ask questions of the mechanics that work in the garage about their work to build your knowledge of auto mechanics.
Read do-it-yourself car mechanic books. Some general ones to start out with are "Auto Fundamentals" by Mark Stockel and Chris Johanson and "How Cars Work" by Tom Newton. Move into more complicated and technical books as you master the basics. Some of these include "Advanced Engine Performance Diagnosis" by James Halderman and Chase Mitchell and "Auto Electricity and Electronics" by James E. Duffy.
Practice what you have learnt on your own car or with spare parts. Car junkyards are good places to get cheap practice parts. If you want to learn to rebuild a carburettor after reading about them, head to the scrap yard to pick one up to practice on. You will not have access to computer diagnostic tools, but taking things apart and putting them back together, or trying to build your own engine out of scrap parts, is a good way to learn how each part functions on its own and in conjunction with others.
Attend formal training courses to become a car mechanic. High schools, community colleges and vocational schools offer certificate and degree programs in auto mechanics. Programs are often a year to two years in length. Find an accredited program that offers career placement or advising services to help you find a job after you graduate.