Write for Discovery.
Just start writing. Who knows where you'll end up?
How To Write A Book Without Going Out Of Your Mind Start by writing, without any preconceived ideas. You never know where you'll end up... your muse knows, however. by Angela Booth: http://angelabooth.com/wp/ Visit my Just Write a Book Blog for more inspiration: http://www.justwriteabook.com/blog/
Lists, mind maps, sketches...
Brainstorming is fun. Allow yourself to become inspired. Relax. Try sketching, and doodling. Let your hands move, without interference.