All Writing Starts with an Idea.
If you're very lucky, the idea strikes hard, in the middle of the night. And you wake and scribble down a story and bang, that's it, you're ready to go. Like me, when I had a dream of a girl, fighting in a forest. (You can read more about my dream, and what it led to here)
There might be a 'what-if' moment:
- What if a whole planet is sentient?
- What if the world was just a computer programme?
- What if a necklace could destroy it's wearer's heart?
If you're short of material, idea generators are available on line or in writing magazines. Tumblr has a few, such as WriteWorld and there are plenty of images on Pinterest
Your idea becomes the premise of your novel. It's the 5 minute plot summary, the elevator pitch.
You can do a lot with a premise; you can elaborate it into several paragraphs, into a short story, into a novel. Randy Ingermanson has an interesting technique called the Snowflake Method. I've played with this concept, and while I quite like it (see my post on Planning Your Novel) I haven't yet extended it from basic premise into full blown plan.
Just remember, that an intriguing idea is only that. You need characters and conflict to make a novel. Also, you need patience.
Finally, some people advise to check there's a market for your idea before developing it into a novel.
Personally, I think: Markets are fickle. Write what you like.