Registering your copyright with the U.S. Copyright Office is not a requirement for copyright protection. However, copyright laws provide several advantages to those copyright owners who take the time to register. A formal registration establishes a public record of your copyright claim. Should someone infringe on your copyright, in order to file an infringement lawsuit, your work must first be registered. With a registered copyright, you're entitled to claim not only actual damages, but statutory damages and attorney's fees as well. And registration will allow you to record your ownership with the U.S. Customs Service for protection against the importation of infringing copies. Registration of works may be done at any time within the life of the copyright.
One last thing to keep in mind: While copyright protection is instantly provided once the work is created and documented, it only protects the particular arrangement of words or the way something looks. It does not protect the subject matter of the information being communicated. If it did, there would only be one book on history, one type of accounting software, or one statue of lovers locked in a kiss. And that would be a sad situation, indeed.