Not sure where to start? the story you want to tell is too big, and you just feel overwhelmed?
Break it down, then break it down some more.
1) Make notes on all the different parts of the story, lets call them chapters
2) Break each chapter down into segments of what is happening at that part of the chapter.
3) Write a rough draft of these small bites.
4) Start gluing the small bites into larger bites and.......
So what you are really doing is just writing small short snippets, and now it's not such a huge task after all.