It couldn't be better.

Dear Teacher.

I usually answer against when someone ask me "How are you?"
I learned to tell them "Fine, thank you and you?" whenever someone ask me previous question.
So these days, I am finding smarter(or natural) answer about it.
But I can't.
It is most difficut question for me.
And furthermor, I am also feeling same one in our class.

Please Introduce some right answer against that question "How are you?".

Sincerly yours~

Dear Jack,

When someone asks "How are you?"  it is perfectly fine to say something like "I am doing well, thank you.  How are you?"  It is very polite to return the question to the person who asks you.  You could also say something like "I am fine, thank you for asking.  How are you?" or  "I can't complain!  How are you"  This last response is less formal, and more of a "slang" response.  If you are not feeling well when you are asked, you can say "I've been better, but I'm okay."  This is also a less formal answer.  

Another way to respond is to bring up something recent from your life.  You could say "I'm doing well.  I recently saw the movie Deadpool and it was quite funny.  Have you seen that movie?"  (Of course, you can substitute another activity instead of a movie, such as a sport you played or a book you read, a new food you learned to cook, or something you're working on at your job)

Thank you so much for your question, and I look forward to our next class!  Keep up the good work :)

-Teacher Rebecca