I'm assuming that you mean a thesis, as in the main purpose or argument of an essay paper.
Your teacher should have provided some materials that will guide you through this process; my first point of advice is to review these materials. While most teachers and educators have a common understanding of what a thesis is, it's possible that your teacher is looking for something specific, and so I would defer to their requirements so that following my advice doesn't accidentally lead you to a poor grade.
Generally speaking a good thesis will be;
- Concise: a thesis should be as short as possible, and only as long as necessary. Boil your argument down to the most clear and direct way of phrasing it that you can, without losing essential information.
- Assertive: you have to actually state a position or argument in your thesis. Think of whether someone might disagree with your statement.
- Appropriate: Your thesis needs to respond to the prompt that was given to you. Students who haven't been paying attention or who wait until the last minute often craft thesis statements that are somewhat off-topic or try to obscure themselves with big vocabulary.
First, review the prompt, and brainstorm. Hopefully you're familiar enough with the material that you have some unresolved questions or strong personal opinions about it; keep these in mind. Then consider which of these things will work with the prompt, and eliminate the ones that won't. Then eliminate the ones that are too complex, such as ones that would require extensive research. See if you can easily combine two or more ideas into one, without making it too complicated. Finally, once you've got an idea for a thesis, you can start writing pieces of phrases that come to mind. Play around with these pieces until you find a phrase that you like, and keep working to fit the pieces together while cutting out the unnecessary bits.
For example: if I was writing a thesis about Alice in Wonderland, I would first consider the prompt. We'll say the prompt asks us to argue whether or not Wonderland is an actual, physical place, or a dream. One of the first things that comes to mind for me is the philosophical challenge to the idea of "reality" itself. I think that I'd prefer to address the assumptions in the prompt, rather than taking a more straightforward approach and trying to prove or disprove Wonderland's reality based on the material in the story. I'll point to the prompt's suggestion of reality, and ask, what makes reality? Was Wonderland "real" to Alice? Did she fear injury or death while she was there? Did she obtain physical needs, or derive happiness from it? Most importantly, what evidence could prove, to an outside observer, that Wonderland was a dream? From this, I would create a thesis statement that goes along the lines of, "Questioning the reality of Wonderland cannot be honestly undertaken without questioning the definition of reality itself, and reality is often a subjective and unscientific experience."
If you mean thesis as the lengthy dissertation/essay involving reasearch as a candidate for a college degree, see the link below
A thesis is one sentence and usually, depending on what your trying to prove, has a similar format. The way I learned to write a thesis was to have a subject, claim, and three points. For example, if you were arguing that texting and driving is dangerous, you would include the subject (texting and driving), a verb and claim (is dangerous), then three reasons why, which is usually the main points in the body paragraphs. You want to make sure your thesis is short and too the point while still giving a good idea of what your essay will be about. Also, make sure that you don't use words like could, would, should, I believe, etc. You want your thesis to sound like a statement (almost like a fact), not an opinion.
In my experience, I learned the thesis as the statement you are trying to prove in your essay. It is more than a main idea in that there is some subjective element to the thesis, and you are trying to convince readers that your opinion is correct. This happens a lot in English and History papers, when you might be proving why a particular scene is indeed the climax of the novel or why those three reasons in particular are why the colonists explored the Americas, just for some examples.
Going back to how you write a good thesis, there are a few "must-have" elements. First, you must address the topic at hand. This includes facts such as what book/event you are talking about and, if necessary, what aspect you are narrowing in on for the paper. Then, you need to state your opinion (what you are trying to prove) stated as a fact. Another optional element (more relevant for English papers) is how whatever you are discussing can be extrapolated to humankind as a whole, like maybe if what you deem the central conflict of the book can be extended to showing jealousy as an intrinsic quality of people. A simplified version of what this could look like is: "In Book A by Author A, the primary conflict is B, showing C." In this case, the parts with A show the facts, B is what you are trying to prove, and C is the greater significance.