First you must decide whether you agree or disagree with bariatric surgery as a weight-loss procedure.
In your thesis statement, you must have two things: first, your opinion and second,your reasons for your opinion.
For example, if you think bariatric surgery is good, you might write, "Bariatric surgery is an excellent way for those who are obese to regain control of their weight issues because it is safe, permanent and uncomplicated."
Your thesis is the LAST sentence of your intro, so before your thesis be sure to write about how awful it is to be morbidly obese. Then, each of the three reasons above will become the basis of a body paragraph.