This is the one sentence in your paper that tells your reader exactly what your paper is going to be about. You should place it at the end of your introductory paragraph.
For a cause and effect composition, you want your thesis to:
Name the observable occurrence you’ll be discussing
Tell your reader what your method of development will be
Are you going to discuss one cause that leads to many effects, or one effect that stems from many causes?
Include either the specific causes or effects that you plan to discuss for the topic you’ve selected.