Montresor is the narrator of the story and he is not a credible or reliable narrator. He is seeking the ultimate revenge against Fortunato who he believes has inflicted,"a thousand insults" of which are never named in the story. While Montresor is building the wall that will seal Fortunato's fate, Fortunato understands what is happening to him, but he thinks it must be a joke because he has no idea why this would be done to him. He begs Montresor to stop, but finally is silent as he succumbs to what his fate may be. It is interesting because as the final brick is placed in position Montresor feels sick. This shows us that he regrets what he has decided to do, but only for a moment because he leaves the catacombs and tells us that no one disturbs the place for 50 years. The moral here then would be that while revenge may be exacting it is not always as fulfilling as one might hope it to be.
The reaction you might have to the story is surprise at the way the story ends because you hardly suspect that Montresor will bury a man alive and laugh in his face while doing it. It might also be to question whether the murder even took place since we heard the entire story from an unreliable narrator in the first place.
The main focus of this story is on revenge. Montresor is getting revenge against Fortunato for the "injuries" that Fortunato delivered upon him. However, those injuries are never explained, and Montresor - speaking 50 years later and still showing cold anger at this man who wronged him - is an emotional and unreliable source of information. We are lead to believe that the revenge was for things that Montresor imagined more than felt. Fortunato certainly gives no sense of having feelings of ill will towards him.
Therefore, the "moral lesson" of the story is that revenge is wrong. The reasons for the revenge might not be enough to warrant what is being done to the victim. Also, since Montresor does feel a need to confess, even 50 years later, and since he does still feel angry - revenge does not help the perpetrator feel any better. Revenge is useless.
Your second question intrigues me. Is your teacher asking for your reaction to the story? If so, you might react to how Montresor committed his revenge, on what you think of Montresor as a person, to what your feelings about revenge are, on whether or not you believe Fortunato deserved what he got, based on what Montresor has told us. Also, do you believe revenge is ok? For more help with the story, I've included a link to the "themes" analysis provided by enotes.