Hello, I just registered and I want to improve my English.

Grammar: “I just register” → should be “I just registered.”

I'm Frencha native usFrench speaker.

Word choice: “French native user” → more natural is “I’m a native French speaker.”

I read some of your histories before sending mine.

I'll would like to be level C2 next week but I know it's a sweet dream.

Sentence structure: “I’ll like to be level C2 next week” → better: “I would like to reach C2 level…”

Hello, I just registered and I want to improve my English.

'Register' should be in the past tense, as it is a completed action

I'm a French native user.

Need to use 'a' - could also say "I'm a native French speaker" or "I'm a native user of French" or "I'm a native speaker of French" - I'm not sure that we 'use' language (in English)

I read some of your histories before sending mine.

I think you might mean 'stories' or 'posts' - histories in French would be (I think! I used Google Translate) the same as "histoires"

I'll like to be level C2 next week but I know it's a sweet dreamlong shot.

colloquialism - "long shot" is something that might be impossible to achieve

Hello, I just registered and I want to improve my English.

I'm not sure about my level,; I suppose it's around B1.

I'm Frencha native usFrench speaker.

I'm not exactly sure if this is exactly what you meant to say.

I read some of your histories before sending mine.

It' was quite fun.

Or you could also say "they were quite fun," if you meant the stories themselves rather than the experience.

I'll like to be level C2 next week but I know it's a sweet dreamunrealistic.

This isn't a natural expression. You could just say that's it's "unrealistic" or "impossible"--or, if you want to say that something you desire is both unrealistic and maybe even a little crazy, you can say it's "a pipe dream."

Hello, I just registered and I want to improve my English.

I'm Frencha native userspeaker of French.

I read some of your historiejournals before sendwriting mine.