A screenplay is a fascinating writing niche. It looks deceptively easy yet it's actually very hard to write a good screenplay.
At the heart of screenplay writing is the question of "a story."
What is a story, within the context of screenplay writing?
If you are writing for Hollywood, you must have a very clear idea of what a "story" is all about.
Not everything that happens around us or we perceive constitute a story.
Here are the 3 characteristics of a good screenplay story:
1) A story must have a protagonist (the "hero," the "person" whose story we are watching). It must be about someone. There are quiet a few European films without a clearly defined protagonist but they are not blockbusters either.
2) The protagonist must have a desire. He or she must be trying to achieve or acquire something very badly. And this should not be a trivial desire either. It should point to a goal that we care about as the audience.
3) A story must have an antagonist, or the "bad guy." Someone or something (could be a non-human entity as well like in the natural disaster movies) must be trying to prevent our protagonist from getting what he or she wants. The antagonist must deny easy access to protagonist's object of desire.
These three rules make up the backbone of what's also known as the "dramatic conflict": the hero desires something but something prevents him or her from getting it.
Here is one classic work on story structure that I think every screenwriter should read before committing anything to paper: