Nice job Hayato!

To make more natural, try not starting every sentence with 'Japan...'. There are different ways to say it that would be less repetitive. For example, in the second and subsequent sentences you could use 'It is/has....' or 'The country is/has...

For example:

It's weather is very humid.

etc

About Japan
I'll introduce (or, Let me introduce you to Japan, or I am going to introduce you to Japan)about you to Japan.

Japan has a population of about 120 million people.

Japan is island country, and it consists of 5 main groups of island; Hokkaido, Honshu, Kyushu and Okinawa.

Japan has four seasons; spring, summer, fall and winter, but in recent years, I think period of spring and fall are shorter than before, because of global warming.

Japan's weather is very humid.

Tokyo is the capital city of japan.
In Japanese, there are three types of written characters, which are kanji, hiragana and katakana, so I think learning Japanese is difficult for non-native people.
The kimono is the traditional Japanese national costume, but these days there aren't many people who wear kimonos in their daily lives.

We take off our shoes when entering a house.

We often use chopsticks when we eat something.
They were introduced from China a long time ago.

Mount Fuji is the highest mountain in Japan, and the it's shape is very beautiful.
It was inscribed on the world heritage list in 2013.