I found this service when I was browsing Reddit.

I found this website when I was browsing Reddit.

I'm a native Japanese nativespeaker studying English because I need to teach it to Jjunior high school students.

We don't often say, "Japanese native" or "Indian native," but we do say things like this: I'm Japanese...I'm Indian...Japanese is my native language...I'm fluent in Japanese...Japanese is my mother tongue.

Don't capitalize Junior or Junior High unless you are naming a specific school. Osaka Junior High School is correct, but a Junior High School in Osaka is incorrectly capitalized. This is also incorrect: I went to a Junior high school in Kyoto.

I found this service when I was browsinglearned about this site on Reddit.

I'll try to write several posts every week.

Great job! It's more common to say, "I'll try to write a few posts each week." That said, there is nothing wrong with what you wrote. Several kind of has a negative meaning often in English. "My boss made me redo the task several times." Not always, but usually, for something positive, we say things like often, or as many times, or a few, or as much as possible.

I'm a Japanese native speaker studying English because I need to teach it to Jjunior high students.

"Middle school" is more commonly said than "junior high," at least here. (US) Not sure about other countries.

I found this service whilen I was browsing Reddit.

This is my first post here.

No need to say here

I'm a Japanese native studying English because I will need to teach it to Junior high students.

I'll try to writepost several posts everytimes a week.

I found this service when I was browsingwebsite on Reddit.

In the US, we'd more often say it this way.