This guide provides you with resources to help navigate the murky waters of copyright, fair use, and permissions.
For your entertainment and education, we also recommend . . .
Reading Jonathan Lethem's "The Ecstasy of Influence: A Plagiarism" from Harper's Magazine, in which every line is plagiarized. A worthy enactment of Picasso's pronouncement: "Good artists copy; great artists steal."
Listening to On the Media's "The Past, Present, and Future of Ownership," which covers, among other topics, why when you sing "Happy Birthday" you are infringing copyright (at least according to the supposed copyright holders of the song) and the reasons one professor encourages his students to plagiarize.