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).
Start off with your overall central topic: in this example, we're starting with flooding. What comes to mind? Major storms that have created flooding here (Allison, Harvey, Imelda), the recovery efforts (FEMA, flood insurance, mold remediation), the possible causes (climate change, land development)...
You're trying to accumulate lots of ideas at this point! Big picture. Make connections, and write whatever comes to mind. When you start getting stuck, turn to Google, Credo Reference, and Wikipedia to get more ideas.
Once you've filled out the map of your topic a bit, look at where you have the most ideas: this is probably the strongest aspect of your topic, and what you should focus your research on.
All those other ideas? We're not going to use them. We want to deeply explore one narrow aspect of the big topic, not try to talk about everything to do with the big topic ever. (That's the job of probably a multi-volume book, not a short essay!)
It's important to still go through this process, though, even if we aren't using most of the ideas, because a) we have to see all this to figure out which thing we're targeting, and b) it still gives us context for how we actually understand the overall topic -- everything is connected! Plus, if we decide we hate our chosen topic, we can come back to drawing board and go another direction easily.
This end result of our mind map is the research topic we'd look into further for the paper: how construction practices affect natural drainage systems and how this could improve Houston's drainage. This is what we research! Based on these ideas, and a bit on our research, we'll figure out questions to ask in that vein (which our research will provide answers for).