First think about historical topics that interest you broadly. Then, gather background information by reading encyclopedias, major books on the topics, and then add focus with a research question.
Example: "How did white and African-American defense plant workers create and think about interracial relationships during World War II?"
WARNING: Avoid selecting a topic that is too broad: "How has war affected sex in America?" is too broad. It would take several books to answer this question.
A good question is narrow enough so that you can find a persuasive answer to it in time to meet the due date for this class paper. A good historical question also demands an answer that is not just yes or no. Why and how questions are often good choices, and so are questions that ask you to compare and contrast a topic in different locations or time periods; so are questions that ask you to explain the relationship between one event or historical process and another.
Connecting your interpretation to previous work by other historians:
Once you have a topic in mind, you need to find out what other scholars have written about your topic. If they've used the same sources you were thinking of using and reached the same conclusions, there's no point in repeating their work, so you should look for another topic.
Most of the time, though, you'll find that other scholars have used different sources and/or asked different questions, and that reading their work will help you place your own paper in perspective. When you are writing your paper, you will cite these historians - both their arguments about the material, and also (sometimes) their research findings.
Example: "As Tera Hunter has argued concerning Atlanta's laundresses, black women workers preferred work outside the homes of their white employers"(and then you would cite Hunter in a footnote, including page numbers).
Adapted from: Brown, "Writing about History."