<p>I do not know much about the French system but here is what typical US students do</p>
<p>1) Take the most rigorous curriculum in Math and Science including AP/IB and possibly even some college classes. So take advanced calculus, not just the basic level.
2) Be involved in science related activities like robotics club, math Olympiad etc.
3) Take part in Science projects and submit to competitions like Intel and Siemens, this includes co-authoring papers, getting internships etc. especially during the summer in scientific labs.
4) Have other activities like music, sports etc.
5) Be within the top 10% of your class or higher. </p>
<p>So to summarize, you need to take the most difficult classes especially in math and science and excel in them, you need to have other activities outside of class that demonstrate your interest and your achievements in science and math. In addition be involved in other stuff that shows you also have other interests. </p>
<p>Others may want to weigh in and it is difficult to describe especially as the French system may have different opportunities. Also not everyone has this type of profile but many do.</p>