You have to write it, completely, and 100% correctly.
Further, you will not get anybody else to write it for you (unless you can pay them thousands of dollars). If they do the writing, you will get a "Story by [you]" credit on the film, they and others will get a "Screenplay by [them]" credit, and the split would be whatever you negotiate.
Movie ideas are a dime a dozen, agents reject literally thousands of scripts a year, and you almost certainly need an agent to get in the door of any production company.
The only chance for an unknown to get anywhere is with a finished screenplay that will captivate people that KNOW screenplays and read them every day. That means no newbie errors.
Some readers (lower rung people whose job is to do the first read of screenplays) wade through a few dozen screenplays a day, and say they reject most in the first two pages. There is no appeal. There is no explanation. They have absolute discretion. The only way to bypass them is to know somebody above them in the hierarchy.
This is a fortress, the only key is a fully completed work people will love. Typically, somebody skilled in writing screenplays and stories has their own ideas and will pursue their own ideas and are not interested in sharing credit with anybody else. Even hearing an idea creates a liability for them of being accused of stealing the idea, whether they did or not.
Which is another common mistake of newbie screenplay writers: Nobody if the film/TV industry will sign a non-disclosure agreement, ever. Readers reject any such attempts to protect an idea, immediately.
For the most part, ideas cannot be protected. Just as one example, consider the central idea of Sherlock Holmes: A hyper-observational, hyper-inference detective. Just like the recent TV series for The Mentalist, Psyche, Monk, and probably a dozen more. All of them almost precisely the same main idea as Sherlock, wrapped in a different package.
Did they copy the idea? Yes! Do they infringe? No!
What is copyright-able in the Sherlock Holmes story is the character names, so shows like Elementary and Sherlock pay for the rights to use "Sherlock Holmes", "Dr. Watson", "Moriarty", "221B Baker Street", and so on, even though they also wrap Sherlock differently (e.g. in modern New York with a female Watson, in modern London).
There are plenty of books on Scriptwriting.
The Complete Book of Scriptwriting [Straczynski]
Writing Scripts Hollywood Will Love [Herbert]
Successful Scriptwriting [Wolff & Cox]
The Scriptwriter's Workbook [Syd Field]
And a thousand more, and another thousand on crafting a story.
The only shortcut is is personally knowing somebody high up in the industry that owes you a big favor, like you carried them out of a burning building yesterday and they still haven't forgotten that.
Added: See answers to this question: How Difficult Is It To Break Into Screenwriting.