Decide on one main statement that you can safely say is the point of the essay you have written or are planning to write. This statement could change, so don't feel as though you have to make a permanent decision right away. Begin with one statement that you feel is your main idea.
Add to the statement by writing about the various proofs that you use or intend to use to support that main statement. For instance, if your main statement is that "the Civil War was a turning point in American history," you'll want to include a continuation such as "because of its impact on civil rights, slavery and the formation of the nation."
Add any conclusion to your thesis statement that you reach in your essay or that you think you will reach in the essay. For the essay about the Civil War, a conclusion might be "therefore the Civil War is an important subject to study in school."
Complete or write your essay, keeping an eye on your thesis statement at all times. It might become necessary for you to tweak your thesis statement. Remember that your thesis statement should be your entire essay boiled down to one statement. If your thesis statement doesn't include something you write about, you'll need to change it. And if your thesis statement includes something that you don't talk about in your essay, you'll need to take that out of the thesis statement.