Вот. Слово Michal можешь заменить на имя друга, а слово Sergei на свое.
Dear Michael,
Thank you for your letter. It’s so wonderful to hear from you.
In your letter you asked me how I spent my summer. I had a great summer. First I went to a youth camp, which is located not far from Kaluga on picturesque lake. We had a lot of sunbathing and swimming in the lake, played soccer, danced in the evening, etc. I made lot of good friends.
Then, the second half of the summer I spent with grandmother in the garden. The garden is located near a large forest. We went hiking, picked mushrooms and berries and bathed in a wide river.
However, I already wanted to school, I missed my school friends.
Sorry, I must go. I will write to you soon again!
All the best,
Sergei