Hi Mazhena,
How long should your letter be? If it's just a few lines, then no problem.
It could start something like this:
Dear (name,)
As I already told you, I would love to be an actress. I've always admired the work of (write your favorite film star here,) and I had dreamed about becoming like him/her since my high school years. Later in my college years I joined a drama group. We presented classical theatrical plays such as (write a couple of your favorite plays here.) My group leader has told me that I have a great talent for acting, and I should study to be an actress. However, my parents think.... (continue from here)
The ending could sound a bit desperate, like: "What do you think I should do? I really feel like acting would suit me the best, but I don't want to upset Mom and Dad. Do you have any advice for me? What would you do in my place?" etc. etc. :))
I know writing essays is always hard, but once you start them, they will get easier and easier.
Желаю много удачи и успехов в Bашем тесте! :)