When you write a song and "fix" it "in a tangible form", i.e. write it down somewhere or record it even in a simple way, your song is at that moment-protected by U.S. copyright laws.
But...PROVING that the song is yours requires another step...REGISTERING the song with the U.S. Copyright Office.
Doing that offers you not just more protection but more proof that the song is yours if you ever have to go to court to sue someone you believe stole your song.
I never cease to be amazed how few artists are willing to spend $40 to register their songs. (even though they brag how 'great' that song may be!).
By the way, these folks are often the same folks who complain about not getting paid to perform their unknown music.
All I know is that when an inventor comes up with some new product that they think will appeal to a certain type of customer, the first thing they do is file for a patent on their invention. The same reaction to protecting songs by registering them should be there for any serious songwriter.
If you really intend to work hard and develop your career as a musician who writes your own songs, don’t wait too long to take care of this simple, but essential task.
Do it now!
Go to http://www.copyright.gov and get started!