In order to start preparing for a career in automotive engineering you'd want to get good grades in math and physics. Although you might not think so, you'd also want to get good grades in English class as well (engineers must communicate their designs and solutions to problems clearly and effectively). In high school you would want to prepare yourself for college and make sure you have all the requirements to enter a mechanical engineering program at a college or university. Once you are in college you might consider joining a FSAE team that designs and builds a race car from scratch to compete against other colleges/universities every year.
In short, to become an automotive engineer you should: study hard, get good grades in all your classes, and study mechanical engineering in college.