Easier said than done. My idea always comes first. My thoughts on what the story will be about, and the way that the story should go. I always ask questions. Who is my MC? Male, female? Age OF MC? Where does he/she live? What's his family like? Both parents? Single parent? Siblings? What could go wrong in his life? This is actually my way of plotting. And there are many more questions to ask. But this helps shape the story in my mind. Interviewing your character is fun too. Asking him the questions actually brings the story more into focus for me. His answers are always better than the answers I think he should have.
And talking to him/her always leads me to his internal problems. The external is easier, but there has to be the inner troubles. That is the meat of the story. The core. Show the character digging deep to understand what he/she is made of. In children's writing that is not obvious. What I mean is, the character growth is written in a way that the reader sees it, but it doesn't seem like a lecture to them. Kids hate a lecture.
That difficult choice that the MC must make shows the growth in a way that we all understand. I force the character to look inside his/herself. Then write the scene. I love writing. All of the brainstorming. The adding scenes and deleting scenes, all to make the story flow like a river. Any thoughts would be very welcome. :)