If a work you wish to utilize is not in the public domain, and not eligible for fair use, you will need to receive permission from a copyright holder.
The first step is to identify the copyright holder. The copyrights of most published works belong to the publisher, rather than the original author. These copyrights are usually marked somewhere on a published work; for example, "© 1997 Universal Syndicate Press" or "copyright by V. Riendeau, 2004."
For some older works, especially photographs and audio recordings, it may prove impossible to identify the copyright holder. These are considered orphan works, and are likely to be eligible for fair use. Always keep records of a search for an orphan work's copyright holder, as they could prove useful if fair use is challenged.
There are a number of websites and organizations that can help you identify and contact copyright holders. Please see the Recommended Resources section for more details.
The second step, once the copyright holder has been determined, is to ask their permission to use protected material. Many publishers prefer a request come by a form on their website, but some authors and publishers may need to be contacted by email, fax or written letter. Whenever possible, it is best to request permission in the preferred format of the copyright holder.