[Welcome to italki!  Your English seems quite good.  Below are some revisions that can be used to express the same information while making it sound more natural.]

My first italki post!

Hey everyone, I just got started here on italki. I'm curious to see how the site works and what new things I can learn.

I'm still testing out the site, so it's possible that I have made some mistakes. Thank you in advance to anyone who has posts corrections!

I have studied english for long time, but I have never had many opportunites to practice writing on the internet.

Regards

My first text in iltalki notebook on italki,

Hi people everyone, I am new in on italki, I wold want like to see how it's it works.

This is my first test notebook, and if some one watch some anyone sees any mistakes in my text writing, please let me know.

I have study studied english for long time, but I have never had the oportunity to practice the write at writing on the internet.

Regards,

Juan *You normally end like this if you're writing in letter format.

My First Text in Italki,

Hi people, I am new in italki; I would like to see how it's works.

This is my first test and if some one watch sees some mistakes in my text , please let me know.

I have been studying English for a long time, but I have never had the opportunity to practice (the)* write writing at on the internet.

Regards

*You don't really need the "the" here. But it does, however, sound better to add "my" instead for more clarity: "...had the opportunity to practice my writing..."