Screenwriting : When should I copyright my script? by Lauren Bartling

You own the copyright the moment you write your script however if you want to register the copyright then you do that with the US Copyright Office (not WGA) and you do it before you make your script public. You can continue to make some changes to the script but major structural changes should call for another registration.

Only when it's as Tight as you can make it. You will no doubt continue re-writing it for some time as you learn your Craft.

The moment you create a body of work, it automatically falls under copyright laws. However, you have to register it with the Library of Congress before being able to bring a lawsuit to court. WGA doesn't actually do anything - they never see the file, so they can't give you support in a law suit about the contents, just the date when you registered it. Library of Congress is the only one that matters.

After I finished the last draft on my first screenplay, I registered it with the Library of Congress. But I only did so after I polished it with the last draft.

Get it the very best that you can, a few rewrites, then register it.

I recommend that once you have rewritten it a few times then to do so