List subjects that interest you.
Think about your past experiences from childhood to now (funny stories, heavy situations, relationships, falling love, breaking up, or things you are aware of that happened to others, news articles or magazine articles that touched you in one way or another...etc
The first book is usually close to the author's heart either a work of "fiction" even it is based on reality with name changes.
Another option is to write about something you know a lot about to the point where you could either teach or offer advice. "How to..."
Lastly you might keep a notebook on your nightstand to record any dreams you might have had which can be turned into a story.
Best of luck!