When I was a kid there were two different men in the neighborhood who worked on there cars in there garages with the door up if the weather was warm, and I would stand around and look into the engine bay of there cars and see what they were doing. I still remember the first time one of them adjusted all the valve clearances.
My grandfather had a 4 car garage with an apartment over it that a family of one of my cousins lived in when I was in high-school. The cousins would buy very cheap cars or sometimes cars that people were ready to junk and get them for free, and we would work on them in that 4 car garage to get them running and fixed up enough to pass inspection. One of the cousins got a part time job at a mechanics garage and after a while he knew more than anyone else. And when I learned to drive we sometimes would fix my dads car. My cousin would say " I will not do the job for you, but I will stay around and tell you how to do it if you need any help." And if something took two people to do it he would pitch in and help then. My cousins and friends worked on our cars in that garage many nights. There was an unwritten rule, that who ever owned the car would pitch in the most for the pizza and pop that we usually got after the work was finished. But if you did not have enough money left over after buying the parts then everyone would pitch fairly equal for the food. Sometimes we would work almost until the sun came up, and sometimes when we were done early in the mornings we would all go to a Perkins restaurant and get breakfast. We did everything and anything that the cars needed. Engines, transmissions, rear-ends, frame welding, brakes, tune-ups, what ever was needed. The cars were much simpler then. Some of them still had points, but most of them had a small solid electronic module for the ignition.
As for first jobs, I returned carts at a grocery store from the parking lot, and set up pins in a bowling alley. I do remember two of those cousins working at gas stations at different times.