A polemic essay is very similar to an argumentative essay: it is an essay where a controversial topic is argued. To begin a polemic essay, I think the best place to start would be to introduce the controversial topic. What is it? What makes it controversial? What are the opinions of the people for and against this? I am not certain as to whether or not your professor wants you to take a stand either way on this topic, but if he or she does, I would probably end the introduction by stating which side of the debate you reside on. I don't know anything about bariatric surgery other than it is used to help obese patients lose weight. So, my introduction would define what bariatric surgery is, and then explain why it is considered controversial (this probably has something to do with the fact that it is potentially life-threatening). The rest of the essay should provide the reader with the facts and opinions from both sides. In the conclusion, restate your thesis.