Speak to your adviser and ask him for some suggestions. Some advisers will help you with a general idea of where your research could go while others will give explicit topics for you to work on. It is up to you as to whether you want to follow the advice on explicit topics or not; you could just use that advice as a starting point. However, you are not going to be passively listening to information from your adviser. You need to ask him questions such as what gaps currently exist in the discourse that you could fill and what kind of research would be best-supported and most viable at your university.
Read up on a topic to make some clear decisions about what you are interested in. The purpose of this reading is to gain a vague understanding of what research has already been done and to better-refine what specific topics you are interested in.
Read everything that's been written on a topic that interests you, based on the results of your reading in the step above. Rather than look for things that interest you, you are now looking in a different direction -- for gaps in research. A good place to focus is in the conclusion of academic essays and books, where possible directions for future research are often discussed.
Make sure that the angle you choose is extremely interesting to you, as you are going to be reading and writing about it for years. So, make sure that reading about, writing about and discussing your topic isn't (too much of) a chore for you. You should be able to discuss your topic with a colleague at length without feeling like you're working.
Make sure that the angle is specific enough that you can focus your research without getting overwhelmed. On the other hand, though, make sure that you can fill five chapters with analysis of your topic.