It all starts with a simple idea you can tell to someone in 1 or 2 sentences. A too complex story/idea will is confusing for the audience and looks and feels like a mess. The idea has to be clear.
Be sure the audience understands what your characters are doing and why they are doing that. Show their way of thinking, but don't make their behaviour predictable (you don't want your story to be boring).
Parts with action and parts with emotions need to be in balance. Action is for creating excitement/thrill, emotion is for understanding.
Don't be affraid to make changes in your story! Every (animated) movie has parts that have never been published and/or changed a lot of times. Keep in mind that every part of the story has to support your idea.
To judge your scenes before you start animating you can use these tips:
- Is this scene only there to connect one scene to the other? If it is, change it to make it interesting. Make it part of the story, not just some filler.
- Is this scene only to explanain something? If it is, change it or even delete it and make changes in other scenes so this scene will become useless. Nothing is more boring than an explaining conversation -_-. If the character has a reason/motivation for his behaviour, no explanation is needed.
- Is the situation in this scene interesting/entertaining? If not, make it interesting or delete it. Not only the characters make a situation interesting/entertaining, the situation itself needs to be interesting/entertaining too. It's a combination of both that makes a scene totally awesome.
- Are the characters coming to life in this scene? If not, make them act like living creatures with real personallities and motivations for their behaviour. What does character A when he ends up in this situation? The character is real and has influence on the situation. Show how a character thinks/feels about the situation. This can change through the movie because the character grows, but make that also understandable for the audience.
- Can this scene be caricatured? If you want realism, please go film your story instead of animating it. It's a waist of your time and a waist of the medium. Animation is ment to be exaggerated, not realistic, but believable.
- How is your story told? Let your story be told by broad cartoon characters instead of the generic charaters. In Snow White the story is told through the dwarfs and the animals, not through Snow White herself, she's too generic to be entertaining.