A suit for copyright infringement must be brought in federal court. In general, you have three years from the most recent infringement to sue. Your copyright must be registered with the U.S. Copyright Office before you can sue anyone for infringement.
A defendant in a copyright action can claim that he or she created the work independently, or that any copying consisted of sections which are not copyrightable, such as facts or news. In addition, a defendant can claim he or she is making "fair use" of the work, which includes criticism, comment, parody, news, research and scholarship.
In determining fair use, the court examines a variety of factors, including whether the use was for profit, how much was copied and the economic effect of the use.