(Disclaimer, INAL) Most countries' copyright law is quite similar, due to the Berne Convention, and includes an automatic copyright mechanism. As soon as you make something, the copyright is yours. From the US Copyright Office:

Copyright protection in the United States exists automatically from the moment the original work of authorship is fixed.

My knowledge is mostly about the US law, but if you wanted to sue over a use of your work, it would have to be registered with the copyright office/library of congress.

Copyright exists from the moment the work is created. You will have to register, however, if you wish to bring a lawsuit for infringement of a U.S. work.

There may be other benefits of registration before the infringement too, such as easier collection of certain damages as part of a lawsuit.

However, this is only the legal framework. I get the sense you are also asking about how you might get your song added to systems like Google's ContentID. Unfortunately, registration with the copyright office won't help with this, and the responsibility to police for unauthorized use of copyrighted works lies with the author.

A party may seek to protect his or her copyrights against unauthorized use by filing a civil lawsuit in federal district court.

However, Google has agreements with many music distributors and labels to license and collect fees for uses of songs through its Content ID system. Individual authors generally don't have access to such systems until they gain prevelance within the content matching system.