At CopyClear we are often asked, "How do 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.
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.
Note. Two or more unpublished songs, song lyrics, or other musical works may be registered with one application and fee. However, the songs must be a titled collection, and they must be by the same author (or there must always be one author common to all the songs). There is danger here. Be sure to read the instructions for registering a collection in Circular 50.