It's important to protect your work when you finish material drafts, which may include a treatment, a first draft, a second draft, and a final draft of a script (across any forms such as screenplay, stage play, radio play, series, comic, etcc.).    Many writers would register each of these.

To "register" a work, it means that you're putting a line in the sand.   You're stating formally that you completed said work with given contents on this date.   

There are two popular places to register work, both completely credible and reliable.  They are ProtectRite and The Writers Guild of America.

ProtectRite

ProtectRite is the creator and pioneer of online intellectual property registration.   Founded in 1994, they act a disinterested third party, establishing a completion date of your registered materials and the security of long-term storage in the event your original is lost or destroyed.  

Writer's Guild of America

The Writer's Guild of America is another option for registering your work.  They've been around for decades and started offering an online registration service circa 2004.

Contact information:

For west coast:

http://www.wga.org/

or mail to:

WGAW Registry

7000 Third Street

Los Angeles, CA 90048

For east coast:

http://www.wgaeast.org/

or mail to:

WGAE

555 West 57th St

New York, New York 10019