Use the Five W's to Focus Your Idea
What - This is your topic. Your passion! Do you have a memory or interest that inspires you? Think about how you could use this situation for a writing assignment.
Who - Narrow by audience. Be more specific with age, race, gender, nationality, etc. Do some people involved with your topic inspire or repulse you? Try looking at the topic from their perspective.
Examples: Instead of just "Vietnam War," try
Experiences of Female Army Nurses in the Vietnam War
Experience of African American enlisted servicemen in the Vietnam War
Vietnam War Protestors and Conscientious Objectors
Where - Narrow by location. Focusing on the topic within a defined region can help avoid information overload. This could be a neighborhood, a city, a state, a region, a country, etc.
When - Narrow by time. Try limiting a larger topic into a smaller time frame.
Why - Narrow by circumstance or method. Does your topic have circumstantial shades of grey that make it hard to define? Try focusing on a single circumstance or method.