A thesis statement should be a short but complete statement that describes what you will talk about in your essay.
A thesis is a statement that tells the reader what you are writing about. If you have a prompt that you are answering, you need to make sure to address it in the thesis statement.
A good thesis statement is going to specifically and completely address the content of the essay. For example, if you are asked to describe a theme for a book in an essay, your thesis would include the title of the book and the theme you are describing.
A thesis for an essay tells the reader what to expect from your paper. In other words, it is a statement or argument that you make at the beginning of your essay. You then use the rest of the essay to prove or explain your thesis.
I am not sure what you mean by "get" a thesis for an essay, but
you would select a thesis by determining what you are going to prove. Whatever argument you wish to make in your essay becomes your thesis.
In short, when you choose a thesis, you take a position on a subject; and then you spend the rest of the essay defending or proving