Once you have a specific, narrowed topic, you'll need to begin generating research questions. To get started the following questions may be helpful.
The illustration below shows one way of developing your research questions. Click on the image for a larger view.
You may also find that once you begin generating one question on a particular aspect of your topic you may have additional ones. For example, the question "In addition to Sanger, who else was involved in the founding?" From that you may be curious about the accomplishments, political positions, experiences of the other members who were involved. And so forth.
From these research questions, you're now ready to search a variety of library resources to find those answers. Depending on what you find, this will eventually lead you to a thesis statement or main argument.