No point feeling timid about this.
After all, I only have to think of it as a rough draft.
Still, you can't waffle on like you can with a bad novel, so some sort of structure feels necessary.
I have adopted Blake Snyder's 'map' or grid to start with, just to give me a shape.
And although selling the damn thing is the last thing on my mind, he makes it very clear that it helps to have the 'logline' down before you start.
He means "the pitch" - and even if you never confront a Hollywood producer, you know what he means when he describes it as the one-liner in the TV Guide film review or the way you describe it to a friend if you want them to come to the movies with you, etc.
Mine doesn't feel quite right yet, but currently I describe it like this:
One White Crow
An optimistically romantic paranormal researcher and his cynical sidekick finally come across a medium who seems to have real powers, but she gets them into trouble with the Psychic Mafia.