According to the U.S. Copyright Office's website, a copyright is an intellectual property law that protects original works of authorship. It protects works such as songs, movies, poetry and novels. It does not protect ideas, titles, slogans or short phrases.
A copyright protects your work the moment that it is created and in a tangible form. As soon as your song is written down or spoken into a device that records your words, you have a copyright that will protect it. You do not have to take further action to obtain a copyright, but you can register your copyright. The U.S. Copyright Office web page linked in the Resources section provides registration information.
Registration is completely voluntary. Even though your work is protected the moment it is in tangible form, registering your copyright can be beneficial for a number of reasons. For example, a registered copyright is helpful if you need to take legal action against someone who steals your work and if you want a public record and certificate of registration.