Research and develop

If you tell people that you want to write films or read a book on how to do it the first thing you will be told is that if you want to write then you have to read. Read anything. Read everything. "You should read the works of Shakespeare" is the advice of many but I know people who will read any book put in front of them but struggle to stick with Shakespeare so my advice is simple. Start by reading something which interests you. Whether it be a specific author, a genre or a novel which has been turned into a film, read something which you think you'll enjoy because if you don't already read a lot then you probably won't be able to start overnight.

Make sure you read slowly. Take every word in. Think about what you're reading and even why those words were used.

Having said that. If you have an idea, let's say for a short film, and you know what it's about and who the characters are and why they're telling your story then the articulation of your idea onto paper may not need the depth of Shakespeare. It probably wouldn't hurt though.

Personally I enjoy reading scripts, or screenplays as they are commonly known. I started reading the scripts of my favourite films but then started reading scripts of films I hadn't even seen yet. (Simply Scripts and Script-O-Rama are good sources)

Speak to people who have experience in whatever it is you're writing about. It is by speaking to these people, or reading their blogs, that you will pick up the little details which you may miss if you are writing about something which you have no experience with. One good example of this is much of the abrasive dialogue of the character Robert Shaw in Jaws, directed by Steven Spielberg. When writer Carl Gottlieb wrote the dialogue he was in fact writing down words which had come from the mouth of a local fisherman. He took real life experience and placed it within his script, into the mouth of a fictional character.

Having said all that, don't feel as though you shouldn't be writing at the same time. If you have an idea start with a short synopsis, just a page (or as much as you feel is necessary, just don't over do it) which covers the arc of your story. Once you have this try to expand it to two or three pages. At this point, and this can be difficult, show people. Not everyone, just a few filmmaker friends and ask for their comments. Using these comments and your continuing research re-write your synopsis and expand it to ten and then twenty pages. You now have your treatment. Your treatment details everything (except dialogue) you have up to this point. Show your friends again then continue the process to research and re-write. Once you are totally happy with your treatment you are ready to begin on your script. Don't worry about how long this takes. You may feel that you don't want to keep working on a treatment when you already have dialogue in mind but with a strong treatment your screenplays first draft should be fairly painless.