Bonjour Bastien! Votre englais est tres bon! Some small edits below, written how I would write it as a native English speaker. General grammer reminder: In English, names of languages are always capitalized.

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What I'm studying

Hi. My name is Bastien, I'm living in France, and I'm twenty three years old.

I'm here for learning on italki to learn English, perhaps Spanish (because I have to do it for the a final exam, but I'm not that motivated to do so). If I'm going doing well, I think that I will start learning a germanic language, like German or Norwegian (or Icelandic, I don't know yet).

So, I'm in the France's French equivalent of high school (called "lycée"), in the scientific section sciences stream. And Because of my section study area, I'm studying several "hard" sciences : life and earth sciences (biology, geology, medicine, anatomy), mathematics, physics and chemistry.

Because of my (probably temporary) health problems, I've "lost" one year, trying to do the work I'm supposed to do. Nonetheless, I've done great things, like learning how to improve my studying habits, how to organize myself, etc., and I've learned a large part of this year's program (especially in life and earth sciences). [this sentence is perfect English!]

My goal is to have a get a good grade at in the final exam, in order to go into a "classe préparatoire aux grandes écoles" in maths, physics and engineering science. A "classe prépa" is a special class where, for two or three years, you have a massive amount of things to learn, with the objective to go of going into a "grande école" (grand school or elite school), like Ecole Polytechnique or Ecole Normale Supérieure, which are really prestigious schools. I want to go to the Ecole Normale Supérieure, for studying to study mathematics and physics. Wikipedia says that our "classes préparatoires" have one of the highest workloads in the world: between 35 and 45 hours of contact per week, including written and oral exams, without the homework.

I think that I'm able to succeed in a "classe prépa", because one of my mother's boyfriend's daughters [I really don't know how to express this, she's is the daughter of the man who is with my mother, but they are not married [don't worry, we also just say "mother's boyfriend's daughters", an easier way to say it would just be "a girl that I know"]] does one of those classes (economics) with fairly good results (she's the fourth in her class, they there are twenty four), and I'm better than her in high school: my grades are better, and I'm studying less than what she did in high school.

I prefer to say "learning", because "studying" often implies "cramming", and I don't do that: it's not the most efficient way to really learn things.