You're probably getting your research into shape about now. After spending weeks buried in libraries, archives, universities, museums, and online resources, chances are you have a pretty solid idea about what your project is going to look like. Maybe you've even picked the color theme for your board. While deciding turquoise vs. teal is fun, don't skimp on the core of your project: the thesis statement.
The thesis holds everything together. In one or two sentences, it tells the audience what you researched about your topic, what you found out, and why it matters. This is a tall order for a couple of sentences, yet the thesis really is the glue that keeps the project focused, narrow and logical. It's your central argument, and it's supported by your research. Everything in your project--everything!--should relate directly to your thesis. If it doesn't, pitch it.
The part of the thesis that is often most difficult to express is what I call the "so what?" question. It's where you make the case for the significance of your research. Why should the audience care about this? Why does it matter? Yet, when you answer that question, you've done what historians strive to do every day: make history relevant, and show how it impacts our contemporary lives.
Check out good blog entries on thesis statements from our friends at Minnesota History Day and New York State History Day. Tobi in New York refers to the Harvard Writing Center's guide to writing a thesis, and that's a good site. Don't rely on online thesis builders to do this work for you. Like wikipedia, they might help you narrow your focus a little bit, but you stand a good chance of ending up with a nonsensical or illogical thesis statement. (I'm not saying don't use them, just approach with caution.) This particular task takes some serious brain work. Good luck!