Take a moment to organize your thoughts. Your ideas may evolve as you start doing some research -- that's a normal part of the process! That makes it all the more important to keep track of where you're going (and what parts of the topic you don't care about).
These are useful tools for laying out your ideas so you can see what the big picture is. Start by doing some free association with your central idea in mind...what do you think of? Go for related themes, expanding the details, or jotting down specific examples you recall. Write down everything. This is the "throw it at the wall and see what sticks" phase.
Try out a little "cell phone research," as in the kind of research you can easily do on your phone: Google, Wikipedia, that kind of thing. This isn't real research yet, so we aren't as concerned with credibility... we're just trying to get some more ideas.
Don't forget, you're not trying to do all-out research for this assignment! You just need to figure out what you want to argue (and what someone might have to say in opposition you can preemptively counter them).