It is easier to narrow your scope to an interesting and manageable paper after you have checked into the background or review of literature on the subject.
One of the best places to get started is an encyclopedia, handbook, or textbook. These tools will provide an overview of your topic and give you hints on ways to limit it. While you are using the encyclopedia, look for an outline at the beginning of the article or the bold-face print and captions within the article. Be sure to note any special terminology, people's names, or events that might be useful in searching for information in other sources.