After I have a basic story idea written down in fragments in my journal that I take with me everywhere, I really start spending the film budget.
The first thing I always buy is a fifty cent notebook dedicated to that film.
I write the story down in this book.
Then I begin to break it down into several lists. I list the characters that are involved.
Then I list locations that will be shown in the film.
A list may include creatures that could possibly appear in the film.
The notebook is super cheap, it can be taken anywhere you go. The lists really let you start to see what is involved in your film. It helps to organize you and keeps everything in one place for you. I know that there are tons of electronic ways to do this, but you would be surprised how nice it is to have simple pen and paper. Plus as an added bonus, once you are super famous, these notebooks will be worth a fortune.
Go ahead, make a mess, have some fun.