Whatever kind of essay you are writing, you want to decide before you finish it what the point will be, where it's going.
Thus you want your thesis statement to express in a sentence what your whole essay says, what you want your readers to know or believe or understand by the end of the essay.
You may have been asked in a previous class to put your thesis statement in the first paragraph of your essay.
There is nothing wrong with putting the thesis statement in the first paragraph, if that will help you to get your point across to your readers.
This doesn't mean that you can only make one assertion in an essay.
But it means that all of the many claims you make must fit together, that they must all support or lead to a single point (claim, conclusion) that defines the whole essay.
first you have your title on what your going to write.
Next you write what your going to write about on a new paragraph that you need to write when it happens and why it happens. Either can be correct depending on the context and the tense. I will write you means the same thing as I will write to you.
A thesis statement, in other words, is only one sentence, not two or three or more. Because the thesis statement is the main point you want to make in one essay; so it should be one sentence.
Frederick Crews defines an essay as "a short piece of nonfiction that tries to make a point in an interesting way." What makes it an essay is that it aims to make a point, one point.