Write the screenplay. Flesh out your ideas by writing them down. If you're not familiar with screenwriting, there are several websites that will lead you through the process.
Create an outline, or develop your idea into a fully completed screenplay. Ideas alone are not copyright protected, but you can legally protect your finished screenplay by registering it online with the Writer's Guild of America (WGA).
Once you've written your script, write a logline--a catchy, one sentence summary of the story that will become a vital marketing tool when it becomes time to pitch your screenplay.
Research the screenwriting industry. Learn about the business by reading film industry publications such as Variety, or pick up screenwriting magazines to educate yourself. If you live in a city that hosts film festivals, attend as many events as you can, to network with people actively working in the field. The more you know about the business, the better your chances of being taken seriously.
Write and send out query letters. Mail them to companies that specialize in your selected genre, such as comedy, action-adventure or horror. Targeting production companies or agencies that best fit your film idea will save you time and effort. Send the producer a query letter. This is a brief note that introduces her to your project. Include your logline, and mention any former film business or writing experience you have. Include your return address, and offer to send the full screenplay as soon as the producer responds with interest.