What the picture needs
The first rule of film scoring is that the picture comes first. Your music can be beautiful on its own, but in a film its job is to tell the audience something the images can't: what a character is feeling, what's about to happen, or what this moment means.
Before you write a single note, watch the scene several times without sound. Ask yourself what the audience should feel at the start, and what they should feel at the end. The distance between those two feelings is the journey your cue has to take.
Then ask what would happen if there were no music at all. Sometimes silence is the strongest choice. A composer who knows when not to play earns the trust of directors and editors very quickly.
The spotting session
A spotting session is where you sit down with the director and decide, scene by scene, where music starts and stops. It's the most important meeting of the project, and it often happens before you've written anything.
Bring a notebook and write down timecodes, but listen most closely to the words the director uses. "Lonely", "tense" or "hopeful" will tell you more about the score than any reference track.
Themes and motifs
A theme is a short musical idea tied to a character, a place or an idea. Introduce it clearly, then let it change as the story changes.
The same four notes can sound triumphant in brass and fragile on a solo piano. That flexibility is what lets a single theme carry an entire film.