начало: Dear Karen..
обсуждение проблемы
решение

Dear Karen,
How are you? Thank you for your letter. I’m writing to give you my opinion on your problem.
To begin with, I understand your wish to go somewhere with your friends, but don’t you think that your parents are right? It is very dangerous to travel alone, especially for girl. And I also agree that such trips cost a lot and they may not want to spend so much money.
Talking about the solution, I’d recommend you to wait for some time. I mean, maybe next year your parents will let you go on your own. Or you can ask your mum to go with you so she doesn’t worry. I believe, it’s a good idea to go for a walk or just spend time together with your friends - that would be safer and cheaper.
I hope my pieces of advice helped you. I’m waiting for your letter!
Best wishes,
[name]