First Step: Construct an overall line of inquiry, a theme or question "How do I come up with an idea?"
Especially if this is your first historical research paper it helps to springboard from an event from your own family's history, perhaps your great grandmother survived the 1918 Influenza epidemic.
Ideas can come from a current historical study that does not explore in depth a specific part of an issue. A chapter on the history of home birth leaves out discussion of how women may have learned about their birthing options in the 1970s.
Comparisons on similar issues between different countries or varying time periods are often instructive. Did Canada react differently to the AIDS crisis then America?