Question

Sprawdzi ktoś z grubsza błędy? :)

Dear Kate,
How are you?I heard about your problem. I know how I can help you.
If I were you, I would talk with your parents. You must say them, that you didn't want moving house. They should understand, that are you having here a lot of friends and, that you want to stay.
I hope I helped you. Please write soon!
Best wishes,

Answer 1

1) I would talk with - I would talk to
2) that you didn't want moving house - that you don't want to move out
3) that are you having here a lot of friends - that you have a lot of friends here
4) must say - have to tell : must odnosi się do odgórnych przepisów i zaleceń, have to badziej do powinności

Answer 2

Dear Kate,
How are you?I heard about your problem. I know how I can help you.
If I were you, I would talk with your parents. You must TELL them that you DON'T want TO MOVE. They should understand that you HAVE a lot of friends HERE and that you want to stay HERE. 
I hope I helped you. Please write BACK soon!
Best wishes,