I have many friends who are songwriters, and "How to Copyright a Song" is a commonly asked question. Let's put this question to rest once and for all.
The good news is copyright actually arises automatically upon an author's or creator's expression of an idea in an original, fixed form (for example, on paper, CD, floppy disk, etc.). In other words, if you write the lyrics to a song on a candy wrapper, it is a 'literary work' and you automatically own the copyright. Another example is if you write the lyrics and the musical notation to a song on a candy wrapper. This would then be a 'literary and musical work', and you would own the rights to both - assuming the work was original and not copied.
According to these examples, we can see it is very simple to own the copyrights to your songs. You simply create an original work and put it in a fixed/tangible form (i.e. the candy wrapper example).
The bad news, however, is unless you can prove you are the original creator of the song, you may run into expensive and time-consuming legal problems defending your work in the event someone copies your work. What this means is, just because you automatically own the copyrights to your song, you can't necessarily prove it.
Fortunately, there are a number of steps one can take to prove you are the copyright owner of the songs.
Firstly, and most importantly, you must put your song into a tangible form (for example, on paper, CD, flash drive, etc.). This is absolutely critical. If you do not have your song in a fixed form, you cannot prove that you own it.
Next, you should register your song with a third party that provides a time-stamped registration certificate. Do not rely on the 'Poor Man's Copyright Method' (i.e. registered mail or emailing yourself) because it is highly unlikely to hold in a court of law. The registration certificate supplied through copyright registries provides protection and proof against infringers, as it guarantees the day and time you submitted your song. It's best to register your work as soon as it is complete to ensure you have the earliest time-stamp possible, and to ensure no one can copy your work before it's registered.
Maintain a copy of your registration certificate in the event infringement occurs. This is a key piece of evidence used to help prove you were the original creator of the lyrics or music. Some online copyright registration services will keep a copy of your work and your registration certificate so you don't have to worry about it.
In the case where multiple people are involved in making the song, keep all documentation/contracts stating who contributed what to the project. This will be important should one member decide (illegally) that they want to sell the song or make their own copy and distribute it.
The above information is meant as a general guide to further your copyright knowledge and does not constitute legal advice. For questions about your specific song, you should consult a copyright lawyer in your country.