Worth noting that different countries/schools/exam boards have different grading systems. A 16/20 on the French Baccalaureate is a very good score, while 80% in the US is what...a B?
Superposed_Cat, here's the deal. If your main hobby is programming, and you're applying to any of the top 20 US colleges, the admissions office will most likely compare you to other kids whose hobby is "programming". And successful (even unsuccessful) applicants to those schools have usually taken their hobby as far as the resources they were allocated with could take them, and then maybe some more. Case in point: the guy who wrote a book about game programming got into Stanford. And MIT's admissions office mentions the guy who built a functioning nuclear reactor who was rejected.
Another example is community service. While showing up every other day to help old people is a great thing, it is insignificant from a "get into a top college perspective", as there are kids who've done community service which had a state or country wide impact.
So, buckle down, pick something and work hard it. But for your own sake. Do it for yourself, not because you think MIT might like it.
The bottom line is you like physics. Guess what? You don't need MIT for that. Just do physics. How? Learn the math (consolidate the basics like trig, geometry, and especially algebra first), and then pick up a book. Heck, use MIT Open Courseware.
At the same time, figure out how to take things to the next level. When you can program C++ and have a code portfolio, e-mail professors/postdocs at local universities and ask if they have a job for someone who's looking for some experience. Don't mention you're in high school and let your code speak for you. I have no experience with that, but that's what I'd do.
Don't get too involved in things you cannot control, like what arbitrary things X school will want with you. A good idea is to just achieve stuff, and then make 10-20 applications (you'll need that many if you're looking for a lot of aid), and let the chips fall where they may.
What you can control is: how you spend your time. Will you spend it debating what ifs or will you spend it studying and exploring your interests?
If I could go back a year or two and talk to my self of then, I would slap him a few times and tell him the same. Other people here had advice along the same lines. I didn't listen. I still got into university somewhere, and I am happy with the outcome. I am also glad on some levels that I didn't listen. I learned a few things. But, if I had been wiser, my current situation would be better.
Anyway, I'll stop here. Regret is not useful after a certain point. Accept reality, and work your way from there.
Good luck.