So we've determined that a story must move. Hero has to move from point A to point B.
But those letters can't represent just any old points. You can't move Hero from the bed to the breakfast table and expect anyone to care. Unless, perhaps, he has to fight a man-eating tiger along the way.
Nor can you move Hero from a better position to a worse position if you want to have a wide audience. Maybe a few gluttons for punishment in the world like to read about the prince becoming a pauper, but most of us prefer to follow along, as the pauper becomes a prince.
Characters need to have strong desires—goals to work toward.
What we really want to do when we're plotting our stories is come up with a character with a strong desire. Only when our character wants something are we able to put stumbling blocks in his way. And we need those stumbling blocks because the obstacle course we set up for our character is the plot. The more he stumbles, the more interesting the book is going to be. The more the poor fellow barks his shins, the more the readers will cry for him and root for him and flip the pages hoping he'll survive, and more than that even, they'll hope he'll be hugely victorious.
This works for all stories. Not just action stories where evil muggle families try to hide the poor relations or where evil wizards who must not be named try to kill everything good and true and loving in the world. It works for quiet books, too.
No matter how quiet the book, every hero needs a goal.
What about the books that don't have kids fighting for their lives? These books still need a character that wants something. I just read a book about a girl who wanted to fit in with the popular crowd at school. The story was not all that compelling because the girl was very rich and very beautiful and if she didn't make friends at her school all she had to do was ask her rich father to transfer her to a different school. There were no negative repercussions threatened if she failed, other than a little bit of embarrassment on her part, perhaps.
What if she had gone to the school on a scholarship and if she failed she would lose the one chance she had to get into a good college? This girl, going into the story, believes she has to make it in the school. She is driven to make it.
We need characters who are driven.
They may be mistaken. The girl may not die if she doesn't get the boyfriend she wants, but if she thinks she'll die the readers will root for her to snag that boy.
So make the desire whatever you want. Our hero may want a basketball scholarship. Our heroine may want a pony. Our orphan kid may want to be adopted. Our rich kid may want a friend. But make that desire the most important thing in the kid's life.
If your rich kid wants to fit in with the popular kids, you have to give him a history that makes us believe the social status he's after is important. Maybe he has a father who drives him and who will only be impressed if the boy hangs with the right sort of children. Maybe our hero has been seeking his father's affirmation all his life. Maybe his father says to him, "You'll never make friends with anyone important—you're too stupid and ugly." Now the kid has to prove his father wrong.
There are all kinds of reasons that a child may need to fit in with the popular kids. The problem with many plots is that the writer doesn't dig deep enough. We figure that all kids want to hang with the popular kids so why do we need a reason?
Our characters have to be strongly motivated if we want to have a good plot. The more motivated the character is the more obstacles we can put in her way, knowing she'll fight like a tiger to get over those obstacles and reach her goal. And that is what makes the story good. The blood, sweat, and tears of our heroes.