What are you going to write about? Ideally, something your are very interested in. Broad categories like "Manga" are too large to effectively do research. What about manga are you going to write: Portrayal of women? The changing aesthetic of mech design in the 1980's? Portrayals of the working class at end of the Edo period? Focusing your topic helps you figure out keywords and actually makes it easier to find sources.
Knowing the correct keywords, or relevant vocabulary is essential to finding what you need. At the beginning of your research your vocabulary will be limited. As you start to do some basic research, you will discover more relevant language that describes your topic. For example, if you are starting to study Geisha, you may not know that the historical district in Edo (now Tokyo) was called Yoshiwara. Knowing the term Yoshiwara may not be obvious when you start to do research on a new topic.
As you think about this topic...what do you already know? After developing the mindmap above, I realize that I'm interested in how women are portrayed vis-a-vis their jobs, especially as Office Ladies (OL's). This makes it easier for me to know what I'm looking for when I investigate potential sources for research. It will also help me form my thesis statement.
If you like, try MindMeister to quickly make your own brainstorming map.
Below is a list of the broad topics you can think about focusing on, you will often cross over into multiple subject areas for your reserach. It's lengthy but will help you realize what topics you can actually write about.