Your thesis statement is the answer to the question or problem raised in your essay.
This question or problem is what motivated you to write your essay in the first place and now, one hopes, will motivate your reader to continue reading.
(Tip: You need to state this question or problem clearly, or else your reader will find something else to read.)
The thesis is the answer or solution to the problem raised; it tells the reader what you plan to show or explain or prove in your essay.
State the subject clearly. This is the general topic about which you will be making some particular claim. For instance, a sample thesis statement in Hacker (C2) would be parsed as follows: "Flextime scheduling" [topic] "should be introduced on a trial basis at the main branch of the Montgomery County Public Library" [claim].
Take a stand. This is the particular claim regarding the above topic for which you plan to argue. It cannot be stated as a question.
Be specific. What exactly are you claiming? An imprecise or vague statement will make it hard for the reader to understand what you hope to demonstrate and to see the connection of that support as it unfolds in the essay.
Express one main idea. Stay focused and keep it simple. Of all that might be said about some topic, what is the one particular claim that you plan to discuss? Similarly, this "one main idea" needs to be manageable: Can you properly support this claim in the number of pages allotted? The thesis must not be too broad.
Be arguable. The claim must be one over which reasonable people might disagree. This requires that there is available evidence that supports your claim (it must be supportable), and your claim cannot simply be some bare fact, where the opposing claim is not arguable because it is simply false.
Try this with your thesis: Could someone plausibly write an essay with an opposing thesis? If not, then your thesis is not arguable.
The claim that "John Wayne was the best actor of all time" is scarcely supportable, for how could we make comparisons across so many different kinds of actors?
Similarly, a bare fact might be quite interesting (for instance, on the topic of the reproductive life of the seahorse, one might note that the male carries fertilized eggs in a pouch until they hatch and are released in a quasi-mammalian fashion), but such a fact is not arguable, nor is the opposite claim.
Be interesting. Your thesis needs to pass the "So what?" test: Why should the reader care about this topic and the specific claim you are making? Either in the larger thesis paragraph or in the statement itself, you need to offer the reader some compelling reason to keep reading.
Be objective. The thesis should not be personal or self-referential; unless it is about yourself, you need to keep yourself out of it. Avoid all mention of "in this essay" or "in my opinion" or "it seems to me." The fact that your name is on the essay indicates that this is your opinion, and repeating this fact will only annoy your readers.