Were a boy finds a book while running away from bullies and finds himself going in the book to another world then he meets a princes in a mountain while he's riding a white dragon and there's a old men and they have a medallion that his sister uses to make her wishes come true. And they are hunting her for it and him as well.

The never ending story? Maybe

Never ending story

I think your actually talking about The Neverending Story III (1994) (http://www.imdb.com/title/tt0110647/)
You should definitely watch the first and second of this set of movies, the third one doesn't really compare in quality.