Hello,
Copyright laws change depending on where you are located. Copyrights in the U.S are completely different from Mongolian copyrights...they still do the same basic things but with small differences and the laws are very different.
I assume you are in the United States. Technically, you already have a copyright. Copyrights are created the exact instant you have created something and as long as you put " (c) March 2006 _________..." ect, then you really don't have to buy or register one for it to hold up in court. BUt if you DO want to file a register for one then you must go through the basic procedures. Mostly, that involves sending in a copy of your song, the fee(s), and filling out paper work.
I am sorry that I can't spell everything out to you, but I am wiped and am aching to go to sleep, so anyother questions may be answerd here:
http://www.copyright.gov/
Oddly enough the site is fairly easy to navigate
Fees:
http://www.copyright.gov/docs/fees.html
THe basics:
http://www.copyright.gov/circs/circ1.html
Hope I helped,
NC