Every story has to have a seed. The vague desire "to write a book" really isn't enough to get you through the first few pages, much less the entire thing.
The good news is, seeds come in all shapes and sizes.
For me the stories usually show up first as a situation. The book I'm wrote for NaNo in 2010 is called The Venus Mirror, and I got the idea a year or so ago while watching a little girl make faces at herself in a mirrored surface. Suddenly I had this idea of a girl who finds a mirror that she does not appear in. She can see her friends and family going about their lives, but there's no sign of her. Why?
I didn't have a character, or a setting, or any kind of plot at that point. I just had a situation that fired my imagination and made me want to explore further. All the books I've written have started that way. What kind of dragon would live in Alaska? What if you died and came back as a cat? And so on.
That really is the only requirement for a seed. It has to be something that grabs you. Just writing what you think you ought to write, or trying to write something because you think it sells, is the surest way to boredom and discouragement.
But as I said, a seed can be anything.
~ Do you have a character in your head that won't stop talking to you? That's a seed.
~ Are you obsessed with a time period from history? Run with it.
~ Do you adore mysteries or espionage thrillers or steamy romances? That love is a seed. Start from there.
You don't have to have all your ducks in a row to write a book. But you do need to start with something that you love, something that makes the book worth writing for you. You need a seed.
What about you? How do your stories start?