Good things to keep in mind writing a thesis statement are concision, clarity and specificity. A thesis is the main idea being conveyed in an essay, usually found in the introduction. Avoid being too vague about the essay topic, and make sure the words communicate the desired message clearly. Do those things while also keeping the thesis between one and two sentences to ensure it conveys the main point.
When struggling with forming an idea for a thesis statement, do a quick free-write on the paper topic. See what ideas and opinions come out of that, and go from there.
Once you decide on the idea, the most important thing about writing a thesis statement is getting the main point of the paper across as clearly as possible without making it too long. It is important to not just give the general topic of the paper but also your position on that topic. The thesis should be one statement that can summarize the argument being made in an essay. It also needs to be interesting and original. For academic papers, professors have to read through so many essays on the same topics; find something new and worth saying.
There are a few different ways to ensure clarity in a thesis statement. One is to know the intended audience. Don't use words that have to be defined later in the essay; use language that your audience can understand. Don't get too technical, yet also avoid being vague and abstract. Make sure the words used in the thesis convey exactly the point the essay is supposed to make. Don't be afraid to be specific.