Here are a few things to think about as you develop your story:
Where does your story start? Do you have an established world or situation that changes at the beginning of the story, or has the change already occurred? What characters are important to setting the scene for getting the action of your story going? How do you want to introduce your readers to what's going on?
What is the climax of the story? How do you get there? What happens immediately after? How do the characters react to what's going on?
How does your story resolve? Does it resolve? What happens to your characters when the story ends? What do you want readers to take away with them from the story? Is there a "moral"? How do you reveal it to your readers without actually stating it?
November is just around the corner. Yes, it may still only be the beginning of October, but time goes quickly, and NaNoWriMo will be here before you know it. What do you know about your story? Is it enough to start writing it? Are you ready for this scary, fun adventure?
*NOTE: NaNoWriMo's website will be relaunching for 2011 on Monday, October 10. This means that it's likely to be a bit laggy as people poke around the site and order their shirts for this year. If you log on, be patient. You can also follow NaNoWriMo on Twitter for updated info about the site and goings-on.