You pass it on to your connections. A screenwriter without connections very rarely makes a sale. Hollywood is the most incestuous industry aside from the mafia. Even porn is more open to outsiders.
So how do you get connections? I hate that phrase: "get connections." Connections are made and maintained, they are something dynamic and living - not a purchasable product.
You start by talking to people, getting to know others who are interested in the business - sites like Triggerstreet help for those of us not in Hollywood. Read blogs by those devoted to the craft and participate in the dialogue in the comments.
If there are local film/screenwriting groups or organizations like a local Screenwriters Association or Women in Film, participate. (There are both in Dallas. I need to join them.) Go to film festivals. And talk to people (a tough one for many writers), get their contact info, follow up a couple of weeks later to say it was good to meet them, and only hit them up after that when you have something relevant to them, such as updating them on what you are doing (provided you are actually doing something worth updating them on).
Otherwise: look up the production companies for films you like that are similar to what you write. (use the Hollywood Creative Directory) Mail them (e-mail or snail mail) query letters asking if they would be interested in reading your script.
Look up the screenwriters for those films, and find out who represents them, and get the contact info for those agents or agencies at least - and contact them
Don't expect to sell your script, so much as to offer it as a writing sample. You should have 3-4 solid scripts (as in ready to produce,) and if told that one is not what they are looking for, you can offer the others. This also shows you're not a one-shotter, and may have the stuff to be a professional writer - so you might get considered for assignments. Most produced screenplays are assignments (and adaptations at that) rather than specs.