Copyright is granted upon fixing your original work in a tangible medium of expression. It applies to published and unpublished works alike. You do not have to register anything. No one else even has to know your work exists. Once it is fixed in a tangible medium of expression, it is protected and you retain the rights until you voluntarily give them away or until the work becomes part of the public domain.
You can register a work with the U.S. Copyright Office—the purpose of which is not to copyright your work, but rather to create a public record of your creation so that you can prove your work came first—but there is really no such thing as a “poor man’s copyright.”
Here it is in the U.S. Copyright Office’s own words:
“The practice of sending a copy of your own work to yourself is sometimes called a ‘poor man’s copyright.’ There is no provision in the copyright law regarding any such type of protection, and it is not a substitute for registration.”
In other words, this is one of the many myths concerning copyright law that floats around.
Disclaimer: I am not a lawyer. The above is based on seminars I’ve taken on copyright law, discussions with colleagues who have studied and/or practiced copyright law, and FAQs available at copyright.gov. This information may be out of date or based on a misunderstanding. It is always advisable to speak to a practicing lawyer.