All original work in a ‘fixed medium’ is automatically copyrighted. However, consider a Copyright License: Creative Commons, for example, makes it easy. With a Creative Commons license, you keep your copyright but allow people to copy and distribute your work provided they give you credit. You can allow them to alter your content or not. In academe, it’s important that we be acknowledged as the author of our works.
However, if you ever want to challenge someone who may have ‘misused’ your work, you should consider filing an original claim to copyright with the US Copyright Office. Registration of your original work establishes a public record of your copyright claim and you will then be able to enforce your rights in federal court.