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.