Foremost make sure that there’s a whole movie. The mistake that a lot of writers make is they have a great idea for a movie, or a book, and everyone defines an idea for a movie or book differently. For example you might say, "I have this great idea about a guy who keeps getting fired from all his jobs, so he starts a business for other people who have been fired." Well, that’s a character, and it promises kind of a direction for a film, and it's got a bit of a concept in it, but it’s not a movie. Or, " I have an idea about aliens in the ocean!" Okay, that's a genre, but what is the story? who are the characters? what do they have to say?
Probably the best way is to write the story out, and then work on the screenplay. Be aware that the structural rigors of writing a screenplay are at the utmost difficulty to accommodate in terms of a writing style versus prose writing. Where I may be able to expound on something in a novel, my writing style must be much more concise if I were writing a screenplay. I'm not saying it's that much harder really, but very different. Trying to find somone to write the screenplay for you will cost you a small fortune. So doing it yourself is probaby the best way to go. I would suggest that having an idea accepted by a studio is longer odds than having the story published as a book. Studio's get hundreds of scripts every day, and it's nearly (not totally, but it seems that way) impossible to have a studio read your script. There are companies out there that claim they will pitch your ideas to the studio's. Caveat emptor on that one my friend.
It's a really tough nut to crack. I have a studio speak to me on average at least once a year. But they are a fickle, and difficult bunch. They usually want their screen writer to change a bunch of things. I'm not into that. I have sold a few things to various production companies and studios. At this precise moment in time none are in production
. But they came to me.
Many production companies actually solicit ideas, so keep a look out for that.
Good luck. Let us know how things progress.