Your first decision is to choose the sub-area of your discipline that you want to focus on. What really interests you? Is it ancient Greece or the French Revolution or modern Korea? Are you interested in political, cultural, or economic history? This doesn't mean a vague, lukewarm interest. It means what are you passionate about? If you are going to spend up to a year studying something, you had better be really interested in it. For some students, this is an easy decisionit's the area covered by the courses you could hardly wait to get into and loved the whole way through.
Once you've settled on a general area of interest, you can start to narrow it down. What fascinates you most about the French Revolutionthe peasants, the bourgeoisie, or the aristocracy? What sources can you locate and use for a study of Roman Egypt? You need to be very practical here. To take an extreme case, no matter how interested you are in studying the political machinations of the Meiji court, it's probably not a smart choice if you are located in Pennsylvania and don't already know Japanese. Access to sources and the ability to use them are important limits to what you can do.
The boundaries for acceptable thesis topics vary but some common principles apply. A thesis topic should raise a question or questions for which answers are not readily apparent. It should allow you to demonstrate your ability to use primary sources and to tackle creatively a problem considered significant in the field. It has to be feasible given the time you have and given the library and other resources that are available.
Don't try to come up with one perfect, well-defined idea all by yourself. A better approach at this stage is to make a list of several ideas that seem plausible to you and then identify a professor who might be willing to direct your thesis. Once you've found a faculty member to work with, he or she will be able to help you evaluate and choose among the ideas you've generated. As an expert in the field, he or she might see possibilities and obstacles that you have missed. Most good thesis topics emerge out of conversations between students and their thesis directors, just as many good topics for the smaller papers you wrote in regular courses grew out of conversations you had with your professors.
If you are having trouble thinking of a topic, it can be useful to remind yourself that you sometimes struggled to come up with a good topic for other papersand yet somehow, after talking with other students and the professor, it all came together in the end. Don't give up too easily or think you have to plan your thesis alone. You are expected to seek help and guidance.