Looking at all of the novels I am currently working on, I try to think back of how I first got the idea for it. The one novel began with just an idea for a character that I liked: A sort of a Bad@$$ demon hunter. That was it; that was the basic foundation for the novel. I eventually began giving him a background and a story and then a conflict to go with it and before you know it, I had an entire plot planned out. And all of it came from the basic idea of a cool character that I randomly happened to think of.
Another novel started with the thought a battle scene I had been daydreaming about in my head while I was on break at work. I imagined a bunch of demonic forces fighting knights with holy powers. Just that basic idea allowed me to expand on it and expand on it and now it has turned into what will hopefully become a four novel series. I imagined why they were fighting and then what were some characters that would have significance in it? What conflicts were there? And so on and so forth.
So take a look at some of your work. What was the first idea or thought that you had that eventually drove you to create the story that you are working on now?