This is very good. Let's go through it and make a few changes.

My first note

Hi everybody,
I'm Marco and I'm 24 years old.

[All of this is perfect.]

I'm an Italian guy, in particular I'm from the North of Italy in a city called Treviso, near Venice.

I'm an Italian guy, from the north of Italy in a city called Treviso, near Venice.

[You don't need the "in particular" if you just continue the sentence with a clause. "North" isn't capitalized here because it's not part of an actual name of the region.]

I want to improve my poor English and I found this site a few weeks ago and now I love it.

I want to improve my poor English, and I found this site a few weeks ago, and now I love it.

[You just need the commas because you are joining sentences together with a conjunction (and).]

I don't knew that there were so much people that want to learn a new language!!

I didn't know that there were so many people that want to learn a new language!!

["Didn't know" because it was before you found italki that you lacked this knowledge. It's an action in the past that happened before another action in the past.]

I found a lot of kind people available to help me, and I also have helped some people with their Italian.

I have found a lot of kind people available to help me, and I also have helped some people with their Italian.

[Just like you used "have helped" later in the sentence, you need "have found" because it is an ongoing action.]

I hope to find new friends and new language partners, so, if you speak english and you want to learn Italian, write me :)

I hope to find new friends and new language partners, so, if you speak English and you want to learn Italian, write me :)

[You just need to capitalize English.]

My first note

Hi everybody,
I'm Marco and I'm 24 years old. I'm an Italian guy, in particular I'm from the North of Italy in a city called Treviso, near Venice. I want to improve my poor English and I found this site a few weeks ago and now I love it. I didn't know that there were so many people that want to learn a new language!! I found a lot of kind people available to help me, and I also have helped some people with their Italian.
I hope to find new friends and new language partners, so, if you speak english and you want to learn Italian, write me :)

My first note

Hi everybody,
I'm Marco and I'm 24 years old. I'm an Italian guy, in particular I'm from the North of Italy in a city called Treviso, near Venice. I want to improve my poor English and I found this site a few weeks ago and now I love it. I did't know that there were so many people that want to learn a new language!! I found a lot of kind people available to help me, and I also have helped some people with their Italian.
I hope to find new friends and new language partners, so, if you speak english and you want to learn Italian, write to me :)