Hello, Andrew. I am an engineer, but I have worked a lot with automotive mechanics. All vehicles are comprised of a variety of systems and components. Get a good understanding of the physics involved - so that you understand combustion, lubrication, heat transfer and fluid mechanics, motion (kinematics), electricity and even some electronics. Another reply suggested the vocational training option, which is a good approach. Every model of vehicle is different, but the basic physics and chemistry have not changed in over 100 years. What has changed is materials, use of electronics and computers, and exhaust emission controls. To keep up, you have to continue to learn. Learn to use your senses to smell, feel, hear what is going on in the machine. Become proficient with basic math and learn to use logic, because very often, you will be trying to find the cause of a problem. It isn't always what the customer or service manager tells you.
Check the simplest things first. Be neat. Be curious and courteous.