Summer

I learned that summer is the most important season for Americans. On In/during the* summer, the streets are full of people walking, exercising or just hanging out.  The bars are crowded, and the parks are busy. In the summer we have they have/there are**  the best festivals, concerts and special outdoor events. In the summer people show up/go out***.
I came come (present tense is better) from a country where it (referring to the weather) is hot most of the year and I thought it was interesting seeing the people here enjoying the hit heat so much. After a winter here I got why summer is so important for Americans. After a long time of cold weather they have the opportunity of spending to spend time outside so they do not waste their time. Summer goes fast; it is just 3 months.
I am loving summer this year, it is way hotter than last summer so I can enjoy all the water activities. Michigan is a great state for you if you like water. There are so many lakes, pounds, and creeks. This year I have done kayaking, paddle boarding, canoeing and I finally learned how to swim.
Now I am going back to Brazil and it will be summer there when I arrive. It will be cool to have two summers this year.

* Usually, we say "in the summer" in spoken language, but "in summer" is also correct as far as I know. 

**Better, since you started out talking about Americans in the third person.

***People show up for something (a party, a class, an event). Speaking generally, people just go out.

Summer

I learned learnt (I am British, learned may be used in American english) that summer is the most important season for Americans. On In summer the streets are full of people walking, exercing exercising or just hanging out, the bars are crowded, the parks are busy. On In summer we have the best festivals, concerts and special outdoor events. On In summer people show up.
I came (come) from a country where it is hot most of the year and I thought found it interesting seeing the people here enjoying the hit heat so much. After a winter here I got why summer is so important for Americans. After a long time of could cold weather they have the opportunity of to spending time outside, so they do not waste their time, summer goes fast, it is just 3 months.
I am loving summer this year, it is way hotter than last summer so I can enjoy all the water activities. Michigan is a great state for you if you like water. There are so many lakes, pounds, creeks. This year I have done kayaking, paddle boarding, canoeing and I finally learned how to swim.
Now I am going back to Brazil and it will be summer there when I arrive. It  will be cool to have two summers this year.

Awesome! Well done, enjoy your two summers!