One of the most common questions asked of writers (especially popular or prolific writers) is "Where do you get your ideas?" The answer, most of the time, is "Everywhere." Writers are inspired all the time by what they see, hear, and experience. Sometimes all it takes is a certain image to get them fired up and typing away. Writers do need to start somewhere, though; but where?
I think the genesis for most ideas for stories can be divided into four categories: character, scene/situation, theme/topic, and setting. Character is starting with either a specific person, say an actual person because you'll write a historical novel; or a type of person, like a cop or a superhero. Abraham Lincoln: Vampire Hunter is an example. You could start with a specific scene or situation. Most of Stephen King's books fall into this section. Salem's Lot is "What if vampires took over a small New England town." Themes are another place to start. Perhaps a book about lost love, feelings of loneliness, or a fictionalised account of cyber bullying. Setting is a particular place, perhaps you want to write a book that takes place in a haunted mansion or on a cruise ship. The book series Star Trek: Titan is an example.
I start with character. I often have a very basic idea for a character that I want to write about and I'll build that character. My story evolves from who that character is and how they came to be that way. My current work-in-progress is The Super School Uniform. When I started, I only had a character in mind: a Japanese schoolgirl with superhuman strength. That's all. Everything else evolved from trying to figure my character out. How old was she; was she a high school student, middle schooler, college age? If she has powers, how did she get them? Was she born with them? Given by aliens? Are they mystical in nature? Is she afraid of her powers or happy and a show-off? By answering questions about my character I was building my story and plot, keeping what worked and discarding ideas that didn't fit.
I often have a specific scene with that character, an image to work from. Sometimes it makes its way into the story, sometimes not. Sometimes an image itself, either of a person or an object, will make me want to write about it. Of course, these are often already licensed or copyrighted, but they get me thinking.
There's no wrong place to start a story. If you are more of an emotional writer, character or theme might work best. Maybe you want to use your favorite vacation spot as a jumping off point for your story. It doesn't matter. Try different approaches. Some are easier than others but they all have one thing in common: they'll get you thinking about a story. And thinking about a story will get you writing it.