Perhaps you wonder, "How can I copyright a song I have written?"
Copyright protection is automatic under the copyright law. An original song you write down or commit to a recording is automatically copyrighted when it is "fixed" in such a written or recorded copy.
However, it is advantageous to register your work with the United States Copyright Office, because copyright registration must generally be made before an infringement suit may be effective.
You can register your work online at https://copyright.gov/registration/.
Form SR is available here.
It will also be helpful for you to have the instructions for Form SR - they are included in Copyright Circular 50 available here.