So, You want to start writing a screenplay but you don’t know where to start. This is what helps me.
First: Don't look at 'formal' structures or theories about how your screenplay should look. That’s not important in the beginning of creating your screenplay. There's only one thing important and that is that you get your story straight first. All other is just an unwelcome distraction from the one point that's important: Your story. What helps me is to first outline the story with the use of ‘low tec’: ‘Post its’.
In my mind I need to know in broad strokes what the story is about. Every significant part of the story gets his own ‘post it’. If possible start with the opening scene description on the first port it and the ending on another post it. Never about how you write it down. Just as long as you (and you alone cause no-one else is important in this phase) can recognize your scene.
Then stick them on a place with enough room to add post its. (I mostly use the back of a flat door) The next step is to fil in ‘the gaps’. So you first start with one big gap between opening and ending. Don’t try to work linear. Just make a ‘post it’ for every scene that comes to mind and stick it onto ‘your door’ around the right place in the story. Takes a lot of shuffling but you’ll get there.
If all goes well you’re story will form itself soon. If not, well I guess you can see that as a sign that your story is not ready enough to be build into a screenplay. So when you have the feeling you have enough post its (for me that’s usually around 30 to 50) I transfer the contents of the post its to Final Draft on the scene cards. (but use any program you like). Then transfer the scene cards to the screenplay and start working out the scenes (this time, linear from beginning to end) with dialogue and action.
It works for me but.. I guess everyone has to find out his own way. Good luck in finding yours.