In the United States, a copyright is established as soon as a work is created. Therefore, a T-shirt creator does not have to do anything to copyright the item's original design. However, there are benefits to registering a copyright through the U.S. Copyright Office. You generally cannot launch an infringement suit in a U.S. court without this registration in place.
Basics of Copyright
Once a T-shirt is designed, the individual who created the design holds the copyright. The exception is when the work is created in a "work made for hire" arrangement, such as in the course of employment. In this case, the employer and not the creator holds the copyright. The holder of the copyright has the legal right to reproduce the design and prevent others from doing so.
You can generally only sue to prevent copyright infringement after the copyright is formally registered. There may be other claims available for an unregistered copyright, however, so it is necessary to speak with an attorney to fully explore your options. You can register a copyright online through the U.S. Copyright Office. The process requires filling out an application form, payment of a fee and depositing the T-shirt design with the office. A paper application is also an option. The Visual Arts Division at the Copyright Office, reachable by phone at (202) 707-8202, provides instructions for the deposit of certain works that you cannot submit electronically.