If your topic isn't assigned, ask yourself these questions to get going:
"Focus" by Michael Dales is licensed under CC BY-NC 2.0
Consider the scope of your assignment - writing about World War II in general is too broad; there's no way to effectively write about all of WWII in a standard sized research paper and make a coherent point. Instead, move to a specific aspect of WWII. For instance, how about focusing on the Battle of Midway, and then going to a specific focus of that battle, such as the role of communications intelligence in the U.S. victory in the battle.
Stating your topic as a question and then turning it into a hypothesis can also help you focus your topic and find supportive research data. A hypothesis is a statement of what you anticipate your research will show. Just like a thesis statement, a hypothesis must be manageable, interesting, specific, and arguable (that is, a debatable proposition that can be proved or disproved by research data).
For example, say you're interested in the effect of music education in grades K-8 on academic achievement, then you might come up with a question such as, "What effect does participation in music education programs in grades K-8 have on individual student achievement in U.S. public schools today?" Once you have a question, then you can do some preliminary reading to learn more about your topic, and then you may propose this hypothesis: "Participation in a music program during grades K-8 can have a positive affect on individual student achievement for students attending public school in the U.S. today."
Here's another example of narrowing and focusing by finding a research question and hypothesis:
"Frida Kahlo" by Guillermo Kahlo is in the Public Domain, CC0
Topic: Frida Kahlo's art
Issue: Influences on Kahlo's art
Research Question: What were the major influences on Kahlo's work?
Hypothesis: The events of her own life were the central influence on Kahlo's work.