No, you do not have to copyright your work unless your school requires you to do so. However, in order to file a suit for infringement you will need to have your copyright registered. We provide copyright preparation services at a cost of $75.00 per document. This is available at the time of submission only. Once we begin to process your dissertation/thesis, we can no longer file on your behalf.
If you wish to file for copyright registration independently, you will need to contact the U.S. Copyright Office at 202-707-6787 or http://www.copyright.gov/help/general-form.html.