Ok, so to find a story you have think ahead. If you are going to write a romance novel, then first think of the characters, roughly design their personalities, what will happen in the book, where this happens (this is mostly optional) and what kind of style you want (horror, happiness, drama etc.)
If you can't come up with the rough layout of these assets, then you have two choices -
1) you can try out and write the book with what you've got, and hope that it'll come to you.
2) find another idea for a book.
Of course, there are more things to think about when coming up with your story, but these things usually do come to your head as you're writing it.
If you were to write about zombies, then you need to think about what is the biggest threat. Is it the zombies? Is it the virus? Is it the people? Or is it the main character him/herself?
You can also easily find an idea by reading other people's books. You might get a spark of an idea, or maybe motivation to continue with your already-existing book!
The possibilities are endless, really. But the process of your writing is usually progressed as you write. So you might get an idea of something that could happen a hundred pages ahead of where you are just when you've started!