As a creator, you have the exclusive right to display and reproduce the original work, as well as make derivative works. This means that no one can copy your work or display it without your permission. You, as the creator, hold the copyright unless the rights have been transferred to someone else, either through a contract, through application of a Creative Commons license or some other method. You do not need to include the copyright symbol - © - in order to receive protection, nor do you need to include a notice of copyright.
When selling a work, the copyright is not automatically transferred to the buyer; a transfer of the rights is typically achieved through a contract or license.
Per the third section of the College Art Association's "Code of Best Practices in Fair Use in the Visual Arts,"
"Useful Articles" are defined as "having an intrinsic utilitarian function." For these types of works, the underlying idea and design are not copyrightable, but particularly creative additions to the underlying concept have the potential to be eligible for copyright. Furthermore, just because something is not covered by copyright does not mean that you cannot protect it; several items in the above list are potentially eligible for either trademark or patent protection. Unlike copyright, however, neither trademarks nor patents are automatically granted to a work upon fixation; both trademarks and patents must be applied for - and approved by - the United States Copyright Office.
For more information on patents and trademarks, see the University of South Florida guide on the subjects. For more information on what is and is not protected by copyright and how to register works, see Chapter 900 of the Compendium (Visual Art Works) and Circular 40: Copyright Registration for Pictorial, Graphic, and Sculptural Works.
Art Copyright, Explained
This guide from Artsy provides a brief overview of who owns the rights to a specific piece of artwork and some famous cases that went to court over artists borrowing work from others without permission.
artrepreneur: art law journal
This site features in-depth but eminently readable articles on all things related to art and intellectual property rights.
Written by the US government, these publications from Copyright.gov provide accessible explanations of copyright and all things related to it.