As we've mentioned, works automatically gain copyright protection once they are fixed in a tangible medium of expression. If you want to specify how your work can be reused, consider applying a Creative Commons License. Creative Commons allows you to customize a license based on different criteria. You can then badge your content with your license, or embed the license on your website. Creative Commons also specifies Best Practices for Attribution.
In cases where it is necessary to obtain permission to use copyrighted works, it's important to identify the copyright holder(s), ask if they still own copyright (that it wasn't a work for hire), or if they have given their copyright to another entity (like the author of an article that signs over copyright to a journal publisher).
Copyright Clearance Center:
The Copyright Clearance Center provides information for those seeking permission to use/ reproduce copyrighted works.
Public Performance Rights:
"Performing" works publicly outside of the classroom requires Public Performance Rights or PPR. This is most common for events like public film screenings. *Even if the film is being screened on campus and is free of charge, PPR is still required. It is not required if an instructor is showing the film to officially registered students in a classroom, where content of film directly relates to course. If you do need PPR, there are companies that manage PPR rights such as Swank or Criterion but these rights are often expensive.
Orphan works are materials that are likely protected by copyright but whose owners are unknown or unidentifiable. Since there is no entity to contact for permissions, reusing orphan works comes with risk since there is no legal solution that makes certain uses permissible. Libraries often come up against this challenge when providing access to and digitizing cultural heritage or archival collections that qualify as orphan works. Learn more about current approaches to this challenges from the U.S. Copyright Office.
If you are not sure whether an original work is available for remix, reuse or redistribution, please contact the the copyright holder for permission or seek legal advice.