Here’s where you go to legally protect your work in the United States. The online process is about as easy as sending a payment through your internet banking. Rates keep rising, so I strongly recommend that you file collections of works, which will protect dozens of works for the same price as a single work. Use Form PA for musical scores, and SR for recordings.
Do NOT use registered mail, a song bank, or any other type of service. They are all scams. Different countries have different approaches to copyright – in some enlightened places, simple online registration of works with a licensing organization equals a strong copyright. But the United States is a special case, especially due to its dominance in the entertainment and media industries. So don’t be an idiot. File a copyright once your work is ready for distribution, publishing, or sale, and especially as the subject of any type of contractual agreement. Though I live in New Zealand, I still lodge works with the US Library of Congress, which automatically protects them internationally under the Berne Convention.