Begin with writing the scenes that you've got a vision of.
Writing the most salient images that you have is an easy way of getting started, or adding to the bulk of your story. Novel in a Week reminded me that you don't have to write in any particular order - you're free to write whichever bits you feel like!
Brilliant! I've found that by writing the scenes that are most potent in my head, my writing has picked up face quite dramatically.
But then as you probably know, writing anything is a good way of getting on with it!