Bonsoir,

Tous les "i" doivent être capitalisés.

"Hello,

[...] fourteen ( pas forteen ) years old, and I'm a teenager.  

[...] I practice the ( pas de "the") musculation, it's my hobbies. ( Comme c'est au pluriel, tu dois rajouter au moins un de tes loisirs ).

I also love to help people. I'm serious and attentive.

If you elect ( pas de e ) I would ( pas will ) put distributors and benches through the HighSchool and expand corridors.

I propose the ideas ( il y a 2 idées ) of having 2 hours to eat and to add more plugs in the establishment to recharge our phone's battery.

If I was the class president, I would help you to find solutions for your problems.  

I want to represent my class and to defend its concepts.

Ce qui donne:

"Hello,

My name is Anna, I am fourteen years old and I'm a teenager. I live in Roisel. I'm friendly, hard-working, honest but I'm clumsy and a bit shy. I practice musculation and... ( mets un autre loisir ici ), that are my hobbies. I also love to help people. I'm serious and attentive.

If you elect me, I would put distributors and benches through the HighSchool and expand corridors. I also propose the ideas of having 2 hours to eat and to add more plugs in the establishment to recharge our phone's battery.

If I was the class president, I would help you to find solutions for your problems. I want to represent my class and to defend its concepts."

Bonne chance!