The thesis states the main argument of your paper. It could read something like: Gun control is necessary for a safe society. Then, you paper supports all the reasons why you believe this to be true (compare Detroit to the city in Canada that lies just north of it, for example. They are separated by a body of water and the former is one of the most violent cities in the world and in the latter, citizens sleep without locking their doors at night). Every argument you make in your paper MUST relate directly to the thesis, so it is OK if you revise it after you have begun writing.
