I believe that to register a script with the Library of Congress you need a final draft (or resend it with the next draft which means another fee, $45 I think...)
I think the WGA is more forgiving though and you can keep submitting multiple drafts without separate registration fees. (Someone please correct me if I'm wrong)
Technically, your stuff is copyrighted already since it's your intellectual property. But if you think you have the next Casablanca written and are nervous about people stealing it you might want to look into copyrights more (I'm just trying to tell you a way to save money on registration fees).
I believe if you do register it with the Library of Congress you can submit later drafts of the work without incurring the fee so long as the material and subject of the work hasn't changed significantly enough to make it classifiable as a separate work. (ie. you can make slight changes, revisions, corrections, but if you want to do something like add a completely new story element or plot-line that changes the outcome, or adds 30 minutes to the length, or some other such comparable change it becomes classifiable as a 'new work' and will require an additional fee.)