I woke up one morning remember a single scene of a dream. I was being chased outside down a long set of steps. On one side was a sheer drop into a body of water, and on the other was a rock face that was too steep to climb--and then I woke up in panic. Why was I being chased? How could I get away from the bad buys I knew were after me? I laid in bed thinking about this for several minutes, letting that scene set into my mind until even after 10 years I can still see it like I'd dreamt it last night. I took that one scene and wrote over a 900 page book. It's since been turned into a paranormal thriller trilogy, but don't you understand? You don't have to know what your whole story is about before you start writing, or even know about your character's deepest reasons for doing what they do before you start that first sentence. Getting down that first frame of a story is what's important.
JUST DO IT!!!!