In your driveway. Seriously. It's where folks with a penchant for automotive engineering and service and modifications find themselves compelled to start. On cars and machines they need or are around them. If you're actually a good candidate for that sort of career or something related, you'll be drawn to it early. It also saves the rest of us from your mistakes, which you will need to experience to gain that skill and understanding. There are scads of places to learn, such as trade schools. I got my start as a porter in a large dealership. As time went on and I made a pest of myself, I was allowed to do simple things like body undercoating or replacing tires. After that, I actually got an assignment that involved breaking into customers' cars who had taken their keys with them or locked them in the car. This got me into trim work. The point is that persistence and learning by doing not only gains you skill and recognition and more opportunities, but it tells you exactly what you like to do. And doing what you like for work is like not working, but playing. And getting smarter. And getting paid.