This was a very interesting start indeed. Now my grammar isn't amazing, so i can't fix mistakes. But i have read my fair share of stories, and i can see that you worked hard on this start. I agree with Burning Ember that celestias sentence was a bit out of chraracter. In my opinion it should be: "Luna, i can see that you are troubled. Do you wanna tell me what is troubling you, while we take a stroll through the gardens?.
The only tip i can give to you so far, is that you have to make sure the characters stay true to their own character. But so far i love it!
Can't wait to see more!