You need a topic
To start with, what questions have other scholars asked about your general topic? If you are writing about something that already has been addressed by other scholars, it can be very useful to survey that literature and ask yourself which approaches are interesting to you.
Often, the best work arises from close engagement with a primary source. As you read, you'll think of questions or begin to shape an argument. The hard part is to find a primary source that addresses the broad general area of interest. Here are some strategies for finding primary sources:
Do not choose a topic for which there is no secondary literature and no primary sources that are accessible to you. Some questions to ask yourself, your advisor, and me:
Talk to your friends about what interests you, and about possible topics. Then ask them to repeat back what they've heard. You may learn something about what's most important to you. And if you can't get your friends or advisor excited about your project, you may not be able to get yourself excited about it, either.
Has some book excited you? Gotten you thinking? What kind of question or topic was that book about?
Truly desperate? Get an encyclopedia that covers the general area of your interest -- say, African American poetry, or the Black Power Movement. You can find a selection in the Trustee Reading Room on the first floor of Firestone, or you can ask me for a suggestion. Encyclopedias rarely break new ground, but they are useful as lists of people, events, and questions that other historians have found worthwhile to study. Skim until you find something that seems interesting, then use the bibliography to get started with the secondary literature.