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.
Take a little time to formally develop your ideas.
You can't really speak coherently about the impact of the automobile on society if you aren't also aware of what the horse-drawn carriage society was like, what the state of manufacturing was like, the purchasing power of the auto's target demographic, and so on.
Scholarly articles are written by experts, for experts. They don't usually take the time to remind their readers what a term or process means, since the assumption is the readers already know. This can also present a barrier to finding those articles, as well. What's a non-expert to do? You've got to learn to speak the language of the field.
Let your research help you do research. As you're learning about the context and picking up on new terminology, you'll also be noticing key people, places, and events that relate to your topic... all of which will help you delve into your deeper research more effectively.
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.
Just remember: your starting points are not your end goals.
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.