Protecting Your Song
Though there are various ways to protect your song and the copyright to it, including mailing yourself a copy and not opening the envelop, but storing it, it is still best to send a copy to the US Copyright Office. Follow the link below to get the particulars. You can also copyright a collection of songs, as well to save some money.
Please consider also joining one of the performing rights organizations to collect royalties on your creations when they get played publically. Click on the links below to explore the advantages and details of each of the major three: ASCAP, BMI, SESAC. Here is a link discussing pros and cons of the three.
Some additional and useful links:
TAXI (song placement)
The DIY Musician (songwriter advocate site)
SoundExchange (collects royalties from satellite radio)
MusicU (Music School for Songwriting, Business, Production, Management, and More)