Create a script, using a scriptwriting program like Final Draft. Most decision makers will not read a script that is not properly formatted. Programs like Final Draft and Moviemaker do much of the formatting for you, leaving you to concentrate on the creativity.
Make sure that the first ten pages of your script hold your reader, and later your audience. Syd Fields, who has extensive knowledge of the film industry, has taught seminars on this subject. Fields makes it clear in "Screenplay: The Foundations of Screenwriting" that a screenplay must engage the reader or the story will end up in a rejection pile. Remember, the first ten pages is generally equal to the first ten minutes of the film.
Get an agent. An agent can shop your script around and has the connections to get a good story in front of the right people. The truth is that frequently you won't even be able to get your foot in the door without an agent. While it might not be fair, some studios will not consider you a professional if you don't have representation. They assume that the agent has vetted your idea before signing you, and thus listening to your idea might be worth their time.
Create a good synopsis. This can help catch the eye of a producer or script reader. There are many scripts and script ideas floating in Hollywood, so it is very difficult to get someone in the industry to even take ten minutes to listen to your idea. This is why you need a well-written summary of your idea that you or your agent can present at a meeting. Be prepared to answer questions about your idea as well. Be passionate, but also listen to their assessment carefully. Even if you get turned down by one studio, you might be able to take the advice or criticism they offer and fine tune your script.
Get on the "inside" of the industry. Go where the movers and shakers are. That may mean getting an internship with a film production company or a job in a casting agent's office. Network, and make connections within the industry. Let people know you have an incredible idea for a film. You will need to be able to show why your idea stands out from the crowd.
Have all parties involved sign a non-disclosure agreement. Your script is considered intellectual property, and you should protect it, even while showing it around to friends or acquaintances. However, keep in mind that it is extremely unlikely that any legitimate studio would try to "steal" your script. If they're interested in producing your script, they'll simply enter into negotiations with you to buy it.