Review your notes from class and from the reading, looking for debatable topics. You might also have had a class discussion that may be worth examining more closely.
Use the textbook if there is one. Review questions at the end of each chapter may signal good essay topics. Also, read the subheadings, which are statements that you might turn into questions. For example, if a subheading says, "Three Causes of the Civil War," you can turn that into a question, "What were the three causes of the Civil War?"
Think about the purpose of the paper. If the assignment is to explain something, "What were the three causes of the Civil War?" will make a good topic. If the purpose is to argue something, you can still turn it into a good topic. For example, if you believe that there was really only one cause of the Civil War and that the other two are merely smoke screens to justify the war, you can argue your point of view.
Ask yourself about pros and cons concerning any topic that looks like a good candidate. If there are only pros or cons, consider whether the idea is too obvious and thus not worth writing about. If you hit on a topic that has both, you should be able to develop at least three main points to support a thesis.
Brainstorm with other students. To do this, sit down with the idea that you will brainstorm together until each participant has a good topic. Write down all ideas, no matter how silly they may at first seem. Sometimes those turn out to be good ideas. For each likely topic, discuss whether you can develop at least three main points.