If you've never written a screenplay, my first suggestion is to watch one of your favorite films and take notes on the following:
What is the mood or genre of the film? Comedy, Horror, Romance, Adventure, War?
How does this scene connect to the next scene?
Note the tension in every scene. Tension is not necessarily "conflict" but it can be.
Who is the Main Character and how does the writer/filmmaker get you (the viewer) to like him? What makes you want to root for this character?
What does the Main Character want? What is her goal? What is the thing that drives her to action?
In Jaws - Brody wants to kill the shark
In Rocky - Rocky wants to go the distance with the champ
In North by Northwest - Thornhill wants to find out who's after him and why
What is the inciting incident? This is the incident that begins the story and there is no turning back for the Main Character. In The Wizard of Oz, the inciting incident was the tornado that lifted Dorothy's house and landed in Oz. Now the story begins - she must get back home (which is her goal). She must find the Wizard of Oz and then she must kill the Wicked Witch of the West (the story/the journey).
How do the supporting characters and events help move the story forward?
Next episode: The Screenplay (Part 2)