Get an agent. The majority of studio and film producers won't even look at an unrepresented screenplay. A good agent knows the marketplace and will attempt to find the right fit for your work. A good place to locate an agent is the Writer's Market. This book is published yearly and contains listings for agents and markets to send writing. Don't send your work to anyone, unless requested.
Send a query letter. Once you've located an agent to review your work, you need to send a one page query letter to the agency, to gauge their interest in representing you. Tell the agent about your screenplay and something about yourself.
Submit your screenplay directly. If you cannot get an agent, or if you don't want an agent, you must submit your screenplay yourself. Check the Writer's Market for listings of those who accept manuscripts. Each listing contains the name of a contact person to address your query to. Once again, submit a query letter, and don't submit a completed manuscript unless requested, or if the market listing says it is okay. Always send materials to the contact person, or your screenplay may never be read.