As a ghostwriter, I find people think that coming up with ideas is the hard part. I'm supposed to do the "easy" part of writing. It's not the easy part at all and many new writers sit down and realize it's not, therefore I make money.
Sit down and write out your script. I'm not familiar with film or anything in that section of writing, but I will elaborate on some of the things other writers have said to me and what I have found from my own search for an agent.
First, sit down and write your script. Undoubtedly, it's going to take you months, so worry about that first. You may give up, so worrying about the other part isn't even necessary.
If you are able to actually finish, go back and edit your work. Perfect it. Many people just can't write. They have no idea what a protagonist is or how to stick with tense. These are the 2 things that annoy agents. It took me about 2 months to go back over my manuscript and fix all the bad writing from a time when I thought there were no rules and I was a genius writer who would be different. Don't think you're going to be a new writer, break all the rules, and you'll be a millionaire. That is not likely going to happen. Read writers.net and read what the agents have to say about writers who have bad writing form.
Now, there is a point where some writers disagree. Some people say that you should never pay for someone to edit your work, but what do you do when you don't know someone who can clean up your script? For some writers, they hire an editor to fix their manuscript. You may know someone who can read through it and help fix grammar errors and let you know if there are any weak points in it.
Then, after you have it alllll shiny and clean, you start to submit to agents. Get ready for lots of rejections! The nice thing about agents nowadays is that they have finally given in and accept email queries. It used to be months before you heard from one, but now you can email and receive feedback within days. Get ready for lots of rejection. You will get lots and lots of "no" answers. I read that Stephen King was rejected 4 times before he finally got an agent. You may have this great script you are proud of, and no agent will take it. Getting an agent is hard. I'm up to 10 rejections out of 30 queries, and I'm close to giving up. To find an agent and their rules for submission, use agentquery.com. This is a great site to find an agent.
Keep writing, even if you are rejected. That is the key. I've also read that agents are taking less clients because of the economy, so it's even harder but this was in reference to book agents, so maybe film is different. Don't give up even if you fail this time around. Many famous, successful writers failed for years before one of their manuscripts was picked up.