The only important error is "doing cold." This is wrong, we don't use "do" that way in English. The most natural phrase would be "it's cold."

Other details:

Capitalize Sul. In English: Rio Grande do Sul. A good source for such things is Wikipedia; in this case, https://en.wikipedia.org/wiki/Rio_Grande_do_Sul

I would break "Hi!" into a separate sentence.

Always capitalize the personal pronoun I.

I'd add in some commas and use a semicolon after "cold."

Hi! Today, here in Rio Grande do Sul, Brazil, it's cold; but I believe that in the afternoon the weather will stay good.

A detail about content. If you are writing to people in the United States, we are very familiar with the river named the "Rio Grande" which forms the border with Mexico. Until I looked it up, I didn't know that Rio Grande do Sul is a state. For a U.S. audience, it would be clearer to add a little more explanation.

Hi! Today, here in Brazil, in my home state of Rio Grande do Sul, it's cold, but I believe that in the afternoon the weather will stay good.

Hi my friends

Hi, today here in Rio Grande do sul, Brazil is cold, but i believe that in the afternoon the weather will stay good.

Hello my friends.

Hi. Today, here in Rio Grande do sul, Brazil is cold, but I believe that in the afternoon the weather will get better/improve.