Take a dated copy of your lyric;
(the date being the date it was origanally written.)
Go to the post office and Mail it to your self :
via - "Certified-Registered" mail.
"Certified"- because your getting a recognized body of the government,
Certifying a date that they handled the material.
And as well it be:"Registered"-,
because it insures that your material is always on the custody of
a government employee,that must sign for the reception of your material,
and is soley responsible for the security of the contents,
and it's delivery,without the contents being mismanaged in any way.
Here's a link to the Copyright office at "The US Library of Congress",
which can provide you with,
the "Formal", Copyright forms,for The U.S.,
and International Copyrights.
"Copyrights"