A good thesis statement would be "Man will go to extremes to get revenge." The whole short story is about revenge, and the easiest thing to write about, therefore, is revenge and how Montresor gets it. Your body paragraphs could discuss Montresor's steps to lead Fortunato to his death. In the conclusion, list the three steps you found to be most important in Fortunato's path to death, and explain how those steps allowed Montresor to get his revenge (in a few words each). Do not use, "In conclusion" to start your conclusion.