To me, you should start by reading as many screenplays as you can. Read the type of work, and the genres, you love and resonate with–and hope to stylistically write. You can Google search for screenplay PDFs, or order them online. Read books about screenwriting, such as Save the Cat by Blake Snyder, Screenplay by Syd Field, and Story by Robert McKee.
Learning about structure is essential, and will ultimately free you creatively once you learn more about how a screenplay is more formally constructed. Write spec scripts. You will probably throw out most of them later on, but that’s okay. You must work, and write, to develop your style. And that will take years. It is an ever-evolving process.
Personally, when I sit down to write a screenplay, I have gestated the idea for a long time. Before I put my fingertips to the keys, I do quite a bit of thinking, walking around in nature, listening to classical music (I have an ever-growing “writing” playlist). I work through beat sheets and outlines.
If I am writing something original, I spend hours upon hours imbuing meanings behind my character names. I have days where I feel like I’ve failed, others when I feel like I’ve succeeded. Over the years I’ve become less afraid to throw bad pages out or start all over again. Once I have a draft, I speak the dialogue aloud, feeling out how it sounds in the air–does it sound truthful, real, authentic?
That whole process can sometimes take a long time–weeks, months, years–every project is different. But once I am ready to write, it feels like a little bell going off. And then there is a settled feeling, and I personally tend to write a first draft quite quickly. After that initial draft is out, however (which is so exciting) the work has only just begun.
From there I re-write like crazy. Later, if I am lucky enough for that project to go somewhere and transform from a hundred or so black and white pages into a living, breathing film or another medium, and I begin working with the Producers, Actors, and other creatives, I re-write again.
Screenplays and stories are ever-changing, ever-evolving, and I love the collaboration that comes from that, especially getting to know the characters once they are alive and in front of me. I enjoy studying speech patterns and idiosyncrasies. And when you’re sitting in that first table read, hearing your words aloud for the very first time–it is truly surreal. All the hard work is worth it.