United States copyright laws automatically protect a writer or artist when her work is "created" (a copy has been made or the work is recorded)---once you have created the work, you automatically obtain the copyright.
Copyright laws have changed. The creator of a work is protected for his lifetime, plus an additional 70 years after death. This applies to work that was originally created on or prior to January 1, 1978.
Register your copyright; this is a legal formality that creates a public record of the copyright and makes it easier prove copyright claims in the event of an infringement lawsuit. Registration forms are available from the Copyright website (see Resources). Mail the completed form, along with the non-refundable fee and a copy of the work, to the address provided.