You need to be fluent in French.
Then you want to make sure you high school diploma will allow you to enter higher education (in this case, a Classe Préparatoire) in France. If not, find out what you can do about this.
There are multiple tracks. The first year is more general than the second. The popular ones for physics and engineering are MPSI and PCSI, with the 2nd year being either MP, PSI, or PCSI (for the ones more interested in chemistry). M = maths, P = physics, C = chemistry, SI = sciences de l'ingénieur/engineering science.
It's a lot of work. You have classes full time. It's just like secondary school, but more intense. It's not like university, where you'll have multiple free blocks between classes. You'll have regular exercise sessions called Kholle, where you'll need to solve problems on the board. They simulate the oral exam conditions of the concours.
Now the concours. After your two years (or 3, if you didn't get the schools you want and want to retake the 2nd year - btw, if you fail the first year, you're out), it's time for the concours. Different concours make different sets of grandes ecoles available to you. You'll have written exams. If you get high grades there, you then do the orals. Then you get a choice of schools based on your rank.
It goes something like that. Bear in mind that I did not attend a classe prépa or a grande ecole. I only strongly considered it. The prépa is very, very cheap. Tuition is free, and if you stay "on campus", it's around 2k euros/year for room and board for the whole year, excluding holidays.
I'm glad I didn't. I'm elsewhere in Europe. It's still a large amount of work, but I have more freedom and control over what I do. But it comes with a price, and that price is that I can't have the other way...i.e...a more structured program where I will have teachers on my back, expecting me to perform. Here, I am on my own.