May be writing a CV and asking for an advice in a chat.

Or may be it reminds me of dancing in school or at home with a group of 10-15 people.

Usually only 2 or 3 of them are really active and the rest stay quiet.

You have many small advices and could not to choose the most valuable or important.

There are many tips and it is hard to choose which ones are the most valuable.

But I tthink there is another thing here .

It helps you not only to improve some simple grammaticsr but also to see different people from different countries and see how they think from their writing.

And tThis part is really valuable

Your sentence is fine, but technically you should not start a sentence with the word "And".

I recommend to combine these sentences to make it natural.

Sometimes it can surprise you and this part is really valuable

So it is, for me, it is more like a cafe in the center of the big city -you never know what or whom you will see today while you drink your coffee.

Where else do you ask a bunch of strangers to give you advice?

Alternatively: "...ask a bunch of strangers for advice?"

May be writing a CV and asking for an advice in a chat.

A single "advice" could be a "piece of advice". Otherwise, you would simply ask "for advice". You might hear "the advice" in a sentence such as "the advice given to me by my teacher was to...", but you'll never hear "an advice".

Something about "in a chat" feels a little odd.

Most tips will be about commas and how to use not using the word “"very” not so often" too much.

I'm assuming this is what you mean. Otherwise, you might change the "not so" to "less" and say "...and how to use the word "very" less often."

Or may be it reminds me of dancing inat school or at home with a group of 10-15 people.

Original is okay (except that "may be" needs to be "maybe"), but this is a little more compact and a bit more natural, I think.

Usually only 2 or 3 of them are really active and the rest stay quiet.

It also reminds me of discussing diet plans with colleagues in the office during lunch breaks.

You have many small pieces of advices and could not toannot choose the most valuable or important.

"could not to choose" sounds a bit nonsensical, I'm afraid. The "to" should be dropped. However, because this should be in present tense (because of "have"), I have changed "could not" to "cannot" as well.

But I tthink there is another something here else.

It helps you not only to improve some simple grammaticsr, but also to see different people from different countries, and see how they think from their writing.

Although not grammatically incorrect, the first "different" can be dropped. To me, "...to see people from different countries..." feels a lot cleaner.

And this part is really valuable.

*Technically*, it is improper to start sentences with "and", but native speakers rarely abide by this rule (or, at least, I don't). That said, you could connect this sentence and the last: "Sometimes it can surprise you, and this part is really valuable."

So it is for me, it is more like a cafe in the center of the big city - you never know what or whom you will see today while you drink your coffee.

"For me" is more natural when it comes as early as possible (but not after sentence starting words like "so", "but", "therefore", etc)