Actually, skip that last one. You don't need to know where your story is going when you start writing; some of us only find out what the story is by writing it.
If you get stuck, Brian Klems has some suggestions for what to write next:
If you find yourself at a loss for what to write next, come up with a way to make things worse, let the characters respond naturally to what’s happening, write a scene that fulfills a promise you made earlier in the book, or work on a scene you know readers will expect based on your genre and the story you’ve told so far.(from 6 Secrets of Writing a Novel Without an Outline)
Brian Klems suggests an organic way of working that I feel comfortable with but the article also gives you a lot of things to think of all at once when trying to write and I'm not sure I can do that. While I'm thinking up stories what seems to work best for me is trying to come up with the story I most want to read. Mind you, sometimes the characters just won't co-operate (I'm going to be editing a novel this month where the main character refused to fall for the attractive woman I'd set up for her and insisted on falling for a character who was supposed to be a minor annoyance).
For anyone doing Nano this year: I'm here to cheer you on. The purple bar is calling your name, just make sure you've got enough chocolate in to fuel your writing. Oh, is it only me that runs on chocolate then?