I Have been in the automotive, heavy vehicle, and off road equipment for about 15 years. I am currently teaching heavy vehicle technology for a school call Advance technology Institute, located in Virginia Beach. While in high school focus on classes like chemistry, physics, and general math will help in the long run. To be the best technician possible you will need to know things like fuel, different end gas emissions and several other things react to each other on a chemical level. It's not A must to be a technician but if you know the chemical process of making bio diesel for an example you will be in a better position to diagnose where the failure point is in the system because of bio diesel. I am in constant talk with several recruiters form large fleet companies like Ryder, Penske, and about a dozen others and do you want to know the main thing they ask me about a particular student? How is his attendance record. They care more about attendance then overall grade. Because they can train and mold you to the way they want the work to be performed only if you show up. Next come in the industry with the understanding that you will be constantly buying tools for the entirety of your career. I currently have about $100,000 in tools and still counting. But if you want to start learning diesels early my biggest suggestion would be to just go buy a diesel instruction manual from amazon. They have several good selections that you can start reading to gain some basic understanding. And if you read something that you do not exactly understand try and contact me on here. I would give my email address but they will not let me on this site. I will be more than happy to try and explain it a little bit better to you.