1) What's a thesis statement?
Your thesis statement is the main idea of your paper, however short or long; thus, whether 3 paragraphs, 3 pages, or 333 pages, your thesis statement is what all of your ideas and information point towards; accordingly, it is the compass of your discussion, and all of your evidence supports your thesis statement.
If your thesis statement is both clear and concise, your audience will know exactly where you stand and what you stand for, making it easy to see how your evidence -- including examples which "show" and explanations that "tell" -- proves your point.
Imagine, then, you are a lawyer in court, a criminal defense attorney. As such, you have a client to defend: your thesis statement. It's your duty and obligation to adequately and articulately defend your client by proving your point, thereby convincing the judge and jury (i.e., your audience) of your client's innocence. A strong thesis statement, therefore, is absolutely necessary (but not sufficient)* to writing effectively.
2. How do I write a thesis statement?
First, you must know the difference between a mere topic and a more comprehensive thesis statement.
The topic of your paper is simply its subject, which answers the following question: what's this paper about?
The thesis statement, on the other hand, is telling your audience about what this paper's about, showing them where you're going with your topic.
For example, if the topic of my discussion is abortion, then my thesis statement will answer the following question: what about abortion?
Am I making an argument? If so, am I in favor of abortion or am I against it or am I ambivalent about it?
Maybe I'm not making an argument at all! Perhaps my paper simply aims to describe and explain the history of abortion in the U.S.
Thus, your thesis statement will also be defined by the purpose of your paper, so you need to know what you're doing and why you're doing it!
*Finally, a strong thesis statement is necessary, but it is not suffient: you must be able to persuade your audience to (at the very least) read your paper. Having successfully persuaded them to do so, then you must successfully make your case, which is to say that you must satisfy the point and purpose of your paper, and to do so you must recognize the role of RHETORIC!
