Constructing your thesis statement is tied with your background research to grapple with the basic assumptions about your topic broadly. As you start to gain a richer understanding of your area of interest you will likely go back and refine your topic and thesis statement several times. Remember, research is cyclical, not linear !
The Craft of Research is an excellent book to read about how to do research well. Chapters 3 and 4 will be especially helpful in formulating your topic and thesis statement. The basic premise is:
Earlier, one of the examples was how Office Ladies (OL) are portrayed in anime and manga, or just popular culture in general. Without a question to guide the research or understanding the significance of the research it can be difficult to focus effectively.
I am studying OL's portrayed in anime and manga because I want to find out if that portrayal empowers women or propogates the bias that women do not advance in business careers in Japanese society in order to help my reader understand certain conceptions of women are reinforced in mass media, an on children from a young age.
When you start to read and do some initial research, you may start to refine your thesis statement. If you discover that OL's are regularly portrayed as overly submissive and inept, than you may focus on that one particular depiction.
There are many ways to narrow or broaden your thesis statement. Be sure to talk with your thesis advisor and librarian to help you refine the scope of your reserach.