My question is: what do you want to do after you graduate from college?
If what you want to do is to become a physician, then remember that there are multiple routes that will get you there. So, while, for some people, the BS/MD route is the best, it is hardly the only route.
You are also pretty young, and there are many years for you to decide, and then change your mind, before you even start college. I taught biology undergrads (so-called “pre-meds”) for a good number of years, and have known many many students who changed their minds about going to medical school halfway through their undergraduates, so they were around 7-10 years older than you when they changed their minds.
So, first and foremost - work hard in high school. Good grades in rigorous classes are helpful for each and every career which you will choose.
Second - look for extracurricular activities which will allow you to observe up close what a medical career is all about, and engage in the type of activities that are related to medicine. So activities like the ones that @Catcherinthetoast wrote about. These will allow you to see if this is a career that you even want. If you do want to be a physician, these will also allow you to engage in this passion for a long time.
The point of extracurriculars is not to make yourself a more attractive applicant. It is to do something that you feel strongly about, but which you cannot do as a student. The admissions people at programs like Brown PLME are not looking at the ECs of their applicants and choosing the applicants with “the best” ECs. They are looking at the applicants’ ECs to learn something about the applicant.
They are looking for applicants who REALLY want to study medicine, and who will most likely not change their mind in the next few years. Engaging for years in extracurricular activities which are related to the work that an MD does demonstrate a deep and abiding interest in medicine.
Finally, as I wrote - a BSMD program is just another route to an MD, and not one that is the most common, either. So, whatever happens, your goal should never be “admission to a BSMD program”, but a career in medicine (or whatever you decide).
Good luck!