Such an exciting career path you are asking about! Mechanical engineering is all about using math and science to design or work on different mechanical or physical systems. Depending on the field you are most interested in I think great steps is to dive in into math classes, physics, chemistry, and any extra curricular that interests you like a FIRST robotics club or maybe come automotive related club.
One thing I'd recommend you is to look into other engineering fields as well. I did recruitment for my college and I usually found out that Mechanical Engineering was like the "default" engineering, cause when people think of engineers they imagine cars, planes, and spaceships. It is important you understand what branch of engineering most interests you and adjust your plan based on that. Civil, Chemical, Biochemical, Computer, Manufacturing, Industrial, and Electrical Engineers all use math and science to solve problems, just think what kind of problem you'd like to solve in the future.
Good luck in your remaining HS years! If you have more specific questions make sure to reach out!
Mechanical Engineering is a large field with lots of great opportunities. I would suggest that you want to do everything you can to continue to work on your math and science coursework, get involved in extra-curricular activities associated with math/science and to see if you can identify some opportunities to talk to and/or shadow some engineers in their jobs. That experience will be invaluable in helping you understand if engineering is right for you and what field(s) you want to focus in.
Best of Luck,
Equally as important is an internal curiosity about “how does that work”, “why did that stop working “, and “what could I do to improve its performance “. That curiosity is then satiated by experimentation and disassembly. Notice: I didn’t say reassembly. My experience has been those with the curiosity tend to graduate as an engineer (not change majors) and excel in performance of their career.