To sue someone for infringing upon your copyright, you must own the copyright to the song. You must be the person who holds the exclusive rights to reproduce, distribute and perform the song publicly or prepare derivative works based on the song. You own the copyright if you are the person who fixed the song into tangible form, if one of your employees created the song as a work-for-hire, or a previous owner transferred the copyright of the song to you. Fixing the song in tangible form can be as simple as writing the music and words on paper. Ordinarily, copyright protection extends from the moment the author fixes the song in tangible form and lasts for her lifetime plus an additional 70 years after her death.
If anyone other than the rightful copyright owner tries to exercise one of the owner’s exclusive rights, a court may find that person has infringed upon the owner’s rights. Typically, to avoid the appearance of infringement, a person wanting to use a song should seek permission from the owner. However, the law has created certain exceptions to an owner's exclusive rights, including "fair use" of a song based on the context or purpose of the use or the effect the use will have on the market or value of the protected work. Courts will evaluate each instance of fair use on its own merits.
Generally, you must register your copyright with the U.S. Copyright Office in order to sue for infringement in federal court. Registration means providing facts about the song and owner(s), filing a copy of the work you want to protect with the Copyright Office and paying the fee. Owners who register their songs before infringement occurs are eligible for additional statutory damages and attorney fees. Without prior registration, they may be limited to the actual damages they can prove.
The U.S. Copyright Office does not enforce copyright laws; instead, it leaves the task of enforcement to the owners and the obligation of asserting defenses to those who want to use the works. If you learn of unauthorized use of your work, you have the right to demand that the person stop using your work. Since copyright cases are dependent on specific circumstances, it may be to your benefit to consult with a lawyer to help you evaluate all of your options, choose the best course of action and navigate the complexities of the federal court system.