Dear Lan, ( bạn có thể thay bằng tên khác )
How are you these days ? Hope you and your family are well. I'm terribly sorry for not writting to you for a long time because of my busy studies. Last week, you asked me about my best friend so today I'll introduce her/him to you. 
Her/ His name is ....and we have been friends for 6 years. He/ She is as the same age as mine, 14 years old and he / she is a student studies in the same school as mine . He/ She is really funny and talkative. He/ She always talks a lot of things to me after school. He is really handsome with short dark hair and black eyes, thin lips. But sometimes he is naughty in the class. He also has many friends at school because he is a sociable person, too. ( Nếu là con gái thì bn viết như này : She is really pretty and cute with beautiful face, long dark hair and blacks eyes. She is a reliable person so she has many friends at school ) He/She will help me when I have a problem that I can't solve, so I always like playing and studying with him / her.
I look forward to hearing from you
Best wishes
.......
Gửi bạn nhé ^^