Once you're finished with your first draft, go ahead and register it with the Library of Congress or the Writer's Guild of America. Both have websites. It's okay if it's only the first draft...better to get it registered the second you're done just in case you happen to share story with someone else and they (unknownst to you) decide to try and do something with it. You can copyright as many different drafts of the same script as you'd like. I'm not sure about the LOC, but the writer's guild charges twenty bucks a script for non-members.
After you've finished your final draft, you have the choice of paying a script analyst to give it a go over. They'll provide you with detailed notes on what's good about it, what works, what doesn't work, and what they think you need to work on. If you choose not to go through a script analyst, here's what you've got to do:
1. Write yourself a logline.
2. Write yourself a synopsis.
3. Write yourself a query letter.
Once you've done these three, it'd help you to do some research, check around which studios, producers or other filmmakers might be interest in what kind of script you've written. For instance, if it's a nasty horror flick you might not want to query Disney...but someone like Lion's Gate would be a good choice.
Then, you wait. No telling how long it will be but what you DON'T do is just up and send your script to any studio, producer or exec. ALWAYS query first.
Hope that helps.
