This is a really good site to help with different kinds of essays. You need to click on Thesis/Support Essays and read everything in that section. There are two sections on writing the thesis statement; the only difference is that you'll phrase yours a a question.
Here's more information on how to write a thesis statement:
Your thesis statement must include factual informationplus your position/opinion/stance. Without your position on the topic, it isn't a true thesis statement. So think of this sentence as the angle you want to take on the topic and what you intend to prove by the end of your paper. (If your statement is simply factual, then there's nothing to prove!)
Read carefully and follow ALL directions.
This is one of the very best places I've seen online to help students write good thesis statements. It shows you sentences that aren't thesis statements and how to turn each one into real thesis statements.