The documentary you chose to write about dealt with an important social issue in our current time. Continue the conversation that was started. Think of a problem facing your school or community that is important to you and propose a solution.
What should your sources do for you?
Mind maps are useful for capturing the connections between your ideas and revealing where you might have discovered more layers of information. You can record ideas going from broad to specific, or vice versa. As you do your background research, continue to take notes on your mind map to help flesh it out.
You can also just start by searching the web for ideas relating to your topic. (Yes, really.) Google is good at natural language, so you can type in questions and misspell things while you're getting a handle on your idea.
Keep in mind that Google doesn't prioritize accuracy in its ranking algorithm, though! A common mistake students make when researching is finding a source that says what they want and therefore concluding it's a good source, when really you need to evaluate the trustworthiness of it first. Pretend the site is someone you don't want to hang out with and you're desperate for a legitimate excuse to get out of it.
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).