Log in to the US Copyright Office to copyright your screenplay. There is an entire online section for screenplays and scripts. Although there will be a fee, it is much cheaper to submit your screenplay online. Online applications can be as low as $35-$55, but paper applications are only $85.
All the information on this website is published in good faith and for general information purpose only. Watson Media does not make any warranties about the completeness, reliability, and accuracy of this information.