Learn how to format a screenplay. Screenwriting is more architecture than literature, so no matter how great you idea is, proper formatting is a must if you want your script to be taken seriously.
Write and refine your script. All Hollywood movies follow the same basic 3-act structure: In Act I, the main character gets up a tree. In Act II rocks get thrown at him. In Act III, he lives and its a comedy, or he dies and its a drama. The more conflict your main character runs into, the better. You hero's motivation is a large part of what makes him likable or believable to audiences.
Hire an agent. No production company is going to read a script from someone unknown. Before you can pitch, you must find a literary agent who likes your script and will take it to potential buyers. An agency list can be found on the Writers Guild of America's web page.
Write a treatment. A treatment is your script in brief: a roughly 10-page rundown of the main plot with no dialogue. It should be enjoyable on its own and read like a story.
Present the treatment to the potential buyers. This is the heart of the pitch. Remember that you are not only selling your idea to the buyers, but yourself as an artist and visionary. If you are not excited by your story, you cannot expect them to be either.
Be calm, professional and concise--the whole presentation should last no more than ten minutes