Dear [Friend's name with a comma]
Thank you very much for your letter. I was really glad to hear from you.
I'll nicely answer your question about subject that I want to offer when I get to Senior Secondary School. As far as I know I always liked doing some logical decisions and I was interested in nature processes all the time of my childhood. So, I want to study chemistry mostly. There are a lot of the secrets of nature among us which we cannot understand with our eyes and we also need special equipment and essential knowledge. Chemistry is a really unique and entertaining science in my opinion which can help us to make a better future.
Well, I have to go now. My mother is calling me.
Hope to hear from you soon.
Best Wishes,
[Your Name without dot]