I never had a class that taught me how to come up with a thesis, either. And that makes no sense--it's such a foundational skill for clear and good writing.

What I want you to do for a moment is forget the writing of the paper. That'll come. For now, think of your thesis as the end point of a process, not the beginning. It's something you arrive at, not start with. ;) And you get there by starting with a broad topic, reading about that topic, narrowing it down, reading about the narrowed down topic, narrowing it down further, reading more, until you've arrived at some QUESTION that you MUST answer, or a debate between scholars that isn't settled that you MUST settle.

So in this case, I'd start by either picking one figure to write about, or maybe some kind of category (ex. black women abolitionists). Maybe you think "okay, I'll compare and contrast a couple of them." (Compare and contrast is a great shortcut to a topic). So you go read about the two people you've chosen. You start to sketch out points where they are similar, points where they are different. In other words--you're doing research!

Eventually, patterns or questions will start to emerge. Maybe the identification of the pattern is your thesis, or maybe you'll reserach up the question, and its answer will be your thesis.

That's how you come up with a strong thesis that will make for an excellent paper--out of research, not before research. :D