Dear friend, (если есть имя того, кто писал письмо, указываете вместо friend)
Thank you for your letter. It was nice to hear from you!
In your letter you asked me about sports. In my country football is the most popular kind of sport. But people also play tennis, ice hockey, volleyball and basketball. Many people like watching sports on TV. I like watching figure skating. It is so amazing! By the way, I sometimes play football and I also enjoy swimming. I often go to the swimming pool with my friends.
What about you? What is your favourite kind of sport? Do you like watching sports on TV? Write back soon!
Best wishes,
(Ваше имя на английском)