Dear friend,
Thank you for your letter. In fact my dream is to become a doctor one day. I like to help people and make them happy, because I become happy too. The most interesting doing for me is studying humans, animals, and all the nature around me, so I'm very interested in biology and chemistry. Also they are very useful for my education.
I need to prepare for my test now so I need to end my letter.
Hope you'll answer me soon,
Best wishes to you
(честно, слова не считала, но надеюсь, что помогла)