If you write a song, it is already copyright protected. What is required of you is that you can prove you wrote it first. Of course you can get official copyright protection but that could occur if there is serious interest in a tune. In Canada all of my songs are registered with SOCAN and ASCAP in the US. Although not officially protected, they at least give a time frame when it was written.. Mailing it via registered mail, to yourself, is generally not accepted by the courts.

Creators of music have a responsibility to their craft. When they have finished using all the notes and words, they must pass them down to the next generation with a simple request. “Use these to create new music.”...Steven McDonald