If you want to publish short fiction, you send it out to magazines. If you want to publish a novel, you send it to literary agencies.
But what if you want to sell a screenplay? Who do you talk to?
There is no right or easy answer to this question. If you asked 1000 screenwriters how they got their break you'd get 1000 different answers but the single continuous thread that will run through every answer is "Write an amazing screenplay." Seriously, the world is always looking for writers and good screenplays will always get noticed.
On a practical level - and assuming you don't know anybody famous and/or well connected - there are a number of things you can try.
1) Write some amazing screenplays (at least 3 or 4, no-one wants a one-hit-wonder they want a 'writer')
*This is actually a bad idea, don't do it.
All of these have worked for someone/not worked for anyone at various times. The key is get out of the house and get out there and eventually you'll make a break for yourself.