Take a part time job in a local automotive shop. Helps you earn money towards college as well as learn what you want to become.
After high school join local community college, which have certification courses and also associate degrees in Automotive technology. Almost all community colleges offer these.
Look for a mechanics on line, locally, network with them and talk to them. Ask for mentoring help and guide you through it.
Be positive, work hard and learn, and success will follow.
Learn some basic finance and personal finance and/or some business management classes as well, that might eventually lead you to start and manage your own automotive business.
but it always should start with taking some studies, so try to find a great training or courses on the subject
and then contact an automotive mechanic shop to train their or contact automotive corporations for an internship.
when you have enough experience take the big step to have your own business.