When you have written a song (or book, or created any other piece of artwork), you automatically create something called a copyright. This is a legal protection that nobody else may use, publish, sell or record your song without your permission. In order to protect your copyright further, you should register it with the U.S. Copyright Office. You may wish to hire an attorney or agent to help you with this process, but it is pretty simple. Most people can do it themselves. There is a small filing fee.
How to Copyright a Song for Free (with Pictures) – wikiHow
