The fact that you’ve written something original and committed it to paper or to a file in your computer means the work is already “copyrighted”—you own it and can prove that you own it. (However, copyright law does not cover ideas . . . nor does it cover titles and pen names.) Your copyright is good “for the life of the author plus 70 years.” However, you can choose to register your book for a fee (see www.copyright.gov). This does give you greater protection in the event of a lawsuit, but it is generally not necessary. Most authors do not opt to register a copyright. However, we all feel more secure about this when we see that familiar © symbol, so all you really need on a manuscript title page is: © [year] + Your Name.
For more information about copyright law, check the Library of Congress website.