Your subject will find you, given the opportunity. It’s very simple. Trust yourself. Just start looking for an action and a character.
When you can express your idea succinctly in terms of action and character—my story is about this person, in this place, doing his/her “thing”—you’re beginning the preparation of your screenplay.
The next step is expanding your subject. Fleshing out the action and focusing on the character broadens the story line and accentuates the details.