Writing a script is the easy part. The trick is getting someone to produce it. Something like 80,000 scripts are written annually in America and only a few hundred of those get made into films, and only a select portion of those get shown in theaters.
Before anyone can sell a script, they have to get it read. The best way I know to get a script read by people who can actually do something with it (aside from having an amazing network, of course) is through e-query delivery services.
ScriptDelivery.com is a great service that I have used before. After sending out a query with them, I got about twenty requests to read my script from agents and producers. I’ve heard good things about SellaScript.com, but I haven’t tried them out yet. Their service sends your query out to over 5,000 agents, managers, and producers.
Just be careful not to get dragged into any scams when using these kinds of services. They send your query to a lot of legitimate industry companies, but they’ll also send it to some hack jobs who will get back to you saying they’ll get your script sold in no time if you just pay them a small (often several hundred dollar) service fee.
But you know better than to fall for something like that.