Here are some general resources to get you started. For more specific information, please ask a librarian.
What's Your Story?: A Young Person's Guide to Writing Fiction
Discusses how to write fiction, exploring such aspects as character, plot, point of view, dialogue, endings, and revising.
Spilling Ink: A Young Writer's Handbook
A how-to guide for upper elementary and middle school children who want to write fiction, including numerous examples.
Just Write: Here's How
The author offers examples from his own experiences on what is needed to become a writer.