A thesis statement tells the focus and purpose of your essay. The thesis clarifies the main argument of the essay and also usually provides insight into the organization of the essay by listing the main subpoints to be addressed. A good thesis statement makes a strong assertion--remember, you are trying to prove a point!
Usually, the thesis statement appears in the last sentence of the introduction paragraph; writers include it in the introduction paragraph, so the reader will understand the purpose of the essay before delving into the main crux of the writer's argument.
If lentzk did not clear it up for you I may be able to put it in simpler words. If you are writing something like a pursuasive essay you would put your thesis statement at the last sentence of your first paragraph stating your three reasons to prove your point on whatever you are discussing. Put your three reasons in the order you want them to be paragraph wise. You explain reason one in the second paragraph and your second reason in the third and so on. I know im only in 9th grade, but I am just trying to help.