Corrections in red.  Stylistics in blue.

First note review

Dear guys, [1]
Here is a revised version of the my first written production here on italki.

" Hello guys, [1] I'm here to try to improve my English language skills in the speaking and writing. I decided to sign up on this site because I think it could be an interesting and very efficient way to do this. If my information is correct, infact, you can practice the English language with other people [1] interested in learning your mother language.  Thus, you can learn while you're building new friendships, and that's really marvelous."

What do you think about this new version? Let me know. In my next writings, I'll try to tell you something about myself (my studies, my hobbies..); writing, obviously, in the most correct way I can.

[1]  The word "guys" is interesting.  When you use "guys" to address a group of people, it can be used for a mixed group of males and females.  So your opening "Dear guys" is correct.  However, when not addressing a group of people, "guys" generally refers only to males.  I assume you would want to practice with either males or females, so I changed the word to "people".