You will work on History Day for months. It will be so much better if you work on a topic that interests you--something you are passionate about. You need to be dedicated and motivated to put the effort in!
You have to do research on your topic. You need to find primary sources as well as secondary sources.
As you consider topics, think about things that will make your work easier:
Look for a topic that took place after the development of the printing press--it'll be a LOT easier to find primary source documents (after 1600).
Look for a topic that that took place at least 25 years ago--it'll be a LOT easier to to show impact.
Look for a local topic--it'll be a LOT easier to access primary source material and find people to interview.
Look for a topic that involves a language you speak and understand. It's hard to do a topic where most of the primary source material is in a language you don't know!
Look for a topic that is broad enough for you to find material.
Look for a topic that is narrow enough that you can stay focused and effective.
Talk to your parents about ideas. Talk to your friends. Go to local history and science museums to get ideas. Go to these pages of the National History Day and the Washington State History Day sites and click on links for more ideas:
Before we go on library field trips, go to each library on-line and do a search for resources. Write down titles and call numbers. For the University of Washington libraries, you also need to write down the location (since they have multiple libraries all over the campus).