Does anyone know where I can go to copyright a script for a commercial?

Thanks!

http://www.copyright.gov/

Peter nailed it. Forms are available on line and the fee for registration is nominal. Note that original expression is protected by copyright as soon as it is fixed in a tangible medium, i.e. when your script was written and saved to your hard drive or printed. Registration is prerequisite to filing an infringement lawsuit and also creates certain desirable legal presumptions, e.g. a presumption of ownership and validity, as well as creating an absolute minimum date for creation.