First, divide the fiction books into different age groups, such as children's, young adult, and adult. Within each group, sort them by author's last name. Also, consider creating sections for popular and classic fiction to make it easier for readers to find what they're looking for.
One way to organize is to create subsections based on the time period the story is set in. For example, historical fiction, contemporary fiction, and futuristic fiction. And label the shelves clearly so that library users can navigate easily.