Browse existing collections of storybooks to see what kinds of topics have been done before so that you can make a story your own. By looking at what has already been written, you can gather ideas and find your own voice. Visit your local bookstore or an online bookstore and browse the current selections.
Consider your style and voice. Do you want your story be in the form of a rhyming poem (such as Dr. Seuss tales or many other contemporary storybooks) or told in prose? Will it be in first person or third?
Decide your cast of characters. Good stories often come from an interaction between characters; knowing your characters can help you to understand the primary conflict. Some common storybook characters are animals, parents, brothers and sisters, school settings and friends. Decide who you will focus your story on and what you want the major conflict to be.
Start with the problem or source of excitement in your story. If you want to write a story about a cat and mouse who are friends, simply start by stating it: "Jenny the cat and Charles the mouse were friends," for example. Then, after you have established your primary point, move on from there and start writing. Although you might not end up keeping your beginning lines, this is a good way to get yourself writing.