The first step of how to sell your screenplay is to write and rewrite so you have a polished screenplay people will want to buy. You may write half a dozen screenplays that face rejection, but at one point you will sell a screenplay. Ensure that the final screenplay you submit is the best you can do. Receiving feedback and rewriting your script accordingly will help tremendously.
It can be challenging to reach industry insiders, but referrals and networking can help you sell your screenplay. Consider the six degrees of separation – that any two people can be connected by six steps of friends of friends. With your connection, and after you have copyrighted your treatment and screenplay, write a query letter that appeals to the potential buyer's needs. It is essentially a sales letter asking them to buy the rights to your screenplay. The director, producer, or film company will need to invest a great deal of time, resources, and money into making your movie, TV show, or film so make sure you've motivated them adequately.
Keep in mind that different financiers specialize in either TV, movies, or film. Furthermore, they will then specialize in a particular genre. Make sure you're marketing to those who are best aligned with your screenplay. You might have the best movie script in the world, but if you're trying to sell your screenplay to a TV show producer, you will likely have poor results.
You should send your treatment along with the query letter. However, do not include the screenplay. Instead, let them know that it is available upon request. You can mail out the query letter and treatment, email it, or even drop off a printed version in person. Wait a few days and follow up to make sure they received it and ask if they have any feedback.