According to the US Copyright Office "Your work is under copyright protection the moment it is created and fixed in a tangible form that it is perceptible either directly or with the aid of a machine or device."
While you do not need to register your work for a copyright, visual artists may do so here:
If you are hosting your works online and have not chosen a CC license, you might want to add a copyright statement. Here are some handy copy and paste statements:
The videos in this playlist are good icebreakers for discussions on intellectual property issues other than than plagiarism, such as copyright infringement, trademark violation and fair use.