Introduce myself
Hi everyone! My name's Ha, I'm from Hanoi, Vietnam. I'm just a secondary school student, so my English is not good much. I not only want to improve my English, but also want to make friend with everybody ^^
I'm learn English because I am a student :) I need write English well in my study. In my country, the secondary school student must to write an essay in their exam on high school, and so do I.
I very like music ^^ Especially J-pop and US-UK :)
Nice to meet everyone in Itaki :)))
P/s: Can you check for me the mistakes?

Question

Introduce myself
Hi everyone! My name's Ha, I'm from Hanoi, Vietnam. I'm just a secondary school student, so my English is not good much. I not only want to improve my English, but also want to make friend with everybody ^^ 
I'm learn English because I am a student :) I need write English well in my study. In my country, the secondary school student must to write an essay in their exam on high school, and so do I.
I very like music ^^ Especially J-pop and US-UK :) 
Nice to meet everyone in Itaki :)))
P/s: Can you check for me the mistakes?

Answer

It's not that bad, you just had few minor flaws in it.

1.) "my English in not that good" would be a better sentence.
2.) "I want to make friends with everybody" you have to put "s" on friends.
3:) "I'm learning English" time doesn't add up.
4.) "I need to be good at English" again, you just have to use the proper way of writing sentences.
5.) "have to" not must to.
6.) "I like music really much" 
7.) "Can you check the mistakes for me?"

I hope I helped a little. You just have to watch some movies in English, or read something in English, because you are not bad at grammar, no, but you have to learn how are you supposed to write the sentences. 
See you around, Ha.