Once you've done some initial background reading, it's time to narrow down your topic to what you really want to focus on. Remember your assignment requirements and consider what you've read thus far.
What aspect of the topic do I want to focus on?
What interests me about the topic?
What do I want to write about?
As you start to narrow this down into a topic/thesis, you'll want to continue to look for more sources. As you research, you might tweak or adjust your topic/thesis. In order to help you find more related sources about your topic, you'll want to identify keywords to help you search.
As you think about what concepts you want to research, think about what particular words might be found in a good article about that topic. For instance, if you are writing about the amusement in the late 1800s changing American Culture. Think of related keywords:
Keywords work best by trial-and-error. Never do only one search. Some keywords will work better than others, and some keywords may lead you to different articles than you found in your first search. Search the databases with the keywords you selected to find relevant articles. And remember to ask a librarian if you need assistance coming up with keywords or looking for sources.