From Writing Forward:
How do you start writing a new story? What is the original idea that germinates into a story? For me it tends to be the theme or some small idea that becomes part of a larger plot. I like this writing exercise because it’s how things start for me, most of the time.
3. Theme Exercise: Universal Ideas
Theme is difficult to explain, but Wikipedia does a good job:
A theme is a broad idea, message, or moral of a story. The message may be about life, society, or human nature. Themes often explore timeless and universal ideas and are almost always implied rather than stated explicitly. Along with plot, character, setting, and style, theme is considered one of the fundamental components of fiction.
I usually think of theme as the big questions that a story asks or its underlying philosophy.
The exercise: Choose a theme and write a list of ways in which a theme can be executed through the course of a story.
You can choose a theme for the characters you sketched in the first exercise or for the three-act structure you developed in the second exercise. For example, in a story where two characters are vying for the same job, the theme might be dream fulfillment (if it’s one or both of the characters’ dream job).
As an alternative, try to identify themes in other stories. Think about your favorite books, movies, and TV shows and make lists of some themes you’ve found in storytelling.