First, however, there are two common misconceptions to dispel. Your thesis is
not an introduction. An introductory paragraph starts with a "hook," which leads into the thesis. You do need an introduction as well as a thesis. Second, a simple restatement of your thesis is not a conclusion. To create that satisfying sense of finality in your conclusion, you must revisit the stuff of your introduction. If you start with a story, return to the story. If you start with a definition, return to the definition, even if only to contradict it.
Sometimes when we write an essay we forget that we're speaking to someone (a reader). We also forget that the beginning of our essay is technically the first impression that we make on the reader, while the conclusion is our last chance to get the reader's attention. Rather than focusing on writing an essay that is simply "correct" (in terms of grammar, following your assignment requirements, etc.) good writers also consider whether or not they've left a lasting impression on their reader. Think about it: the movies you've seen and the books you've read, the ones that really stand out in your mind, probably had an intriguing opening and a compelling ending. Your essay topic may not be as exciting as your favorite movie, but that doesn't mean you can't make sure that your ideas stand out in the reader's mind.
The function of an argumentative essay is to show that your assertion (opinion, theory, hypothesis) about some phenomenon or phenomena is correct or more truthful than others'. The art of argumentation is not an easy skill to acquire. Many people might think that if one simply has an opinion, one can argue it successfully, and these folks are always surprised when others don't agree with them because their logic seems so correct. Argumentative writing is the act of forming reasons, making inductions, drawing conclusions, and applying them to the case in discussion; the operation of inferring propositions, not known or admitted as true, from facts or principles known, admitted, or proved to be true. It clearly explains the process of your reasoning from the known or assumed to the unknown. Without doing this you do not have an argument, you have only an assertion, an essay that is just your unsubstantiated opinion.