Hi Jorge! Great question! You should be proud of yourself for already asking these questions in the 9th grade!
I would highly recommend taking as much math and science classes as you can in high school - ideally calculus, physics, and chemistry (biology is less important for mechanical engineering).
Engineering curriculum early in college can overlap with high school AP classes. It is comforting to see familiar material in college. If you are feeling really good with particular courses it is also nice test out of class to clear up time for other classes or activities that more aligned with personal interests. For me, I was able to test out of several classes which clear up time in my senior year of college to take graduate level acoustics classes. These acoustics classes helped to set me up for my first job out of college.
As you progress in college with mechanical engineering, you get into a lot of physics (Statics and Dynamics), thermodynamics (engines, refrigerators, fuel cells, even photovoltaics in the upper levels), mechanics of materials (when do things break), fluid dynamics, basic electronics, control systems, and heat/mass transfer. As a mechanical engineer, I have a basic understanding of how everything works -that's a broad statement, but I think it's fundamentally true.
I can confidently say that having a strong base in these subjects will set you up for success in college and beyond. Confidence in math, science, and scientific reasoning is helpful in and outside of the classroom.
Also, if you school has any engineering classes or clubs, they would be a great way to confirm your interest in engineering early.