It already is copyrighted! what you need to do is register it, if you want actual cash back from other people recording it or playing it for payment. Here in the UK we're registered with PRS, which was a one off payment for life and anyone who records it (and registers with MCPS etc) gives us a couple of pennies from every CD.

As I said in starting as soon as write the work, you own all rights to the song/music and the only right to alter it etc. What you may need is proof that you wrote it and when you wrote it, if a case ever went to court. This can be done in many ways one of the classic ways is to send a copy to yourself in a sealed envelope via registered mail Don't open it just lock it away and keep it safe, then if ever it's needed as evidence in a case of ownership you've got it.

If you want to be the first person to record the song you can register this as well, which will in effect stop anyone (who attempts to register the request to record the song with MCPS etc) getting permission for the CD pressing.

as NIKE say's Just Do It :-)

PB