of Amontillado," your thesis will be quite different than if the prompt is to analyze a theme of the text.

However, I think that whenever you are looking to develop a text-dependent essay, it is useful to go to the textual evidence first and look for patterns in language or content within that text. It obviously helps if you have annotated the text as you've read to help establish those patterns. Often, the text itself will bring a thesis to light.

Once your pattern of textual evidence is identified, your thesis statement might be phrased something like: "'The Cask of Amontillado' clearly asserts that..." For the organizational aspect, I always think it is helpful to go in chronological order of the textual evidence.

Start with a thesis statement.  A good thesis statement is not a statement of fact.  It needs to be your opinion about the story.  It also needs to be an opinion that is debatable AND defensible.  

A good way to start that kind of thesis statement is with the word "although." That subordinating conjunction allows you the opportunity to admit to any counter arguments that your reader may have, and then provide evidence as to why your opinion is the correct one. For example: "Although Montressor's hurt pride allows him to feel entirely justified in his murder of Fortunado, he still goes to great lengths to hide the crime."  

That thesis statement allows you discuss Montressor's character as well as the events leading up to the murder. It also gives you a wide opening to point out all of the little details that Montressor had lined up in order to get away with the murder in the first place. 

You have asked more than one question and thus transgressed enotes regulations. I have therefore edited this question to make it more general about the story as a whole. Remember you are only allowed to ask ONE question per day in each group.

Poe was a master of writing stories of horror and revenge, and as a "Dark Romantic" the focus of so much of his work is the dark side of humanity - the capacity of us all to commit heinous acts. What is key to note about his work is how he uses masterfully the first person narrator to reveal deeply disturbed psyches and characters who often are unreliable narrators - in that we as readers can see that often there is more going on in their account than their words at first indicate.

This short story is of no exception - consider how it begins:

The thousand injuries of Fortunato I had borne as best I could; but when he ventured upon insult, I vowed revenge.

As the story progresses and we see in particular the friendly way in which Fortunato responds to the narrator, we begin to doubt the veracity of the narrator's comments - would Fortunato really entrust himself if he had insulted the narrator?

As the story moves both its characters and us as readers to the labyrinthine catacombs and underground darkness of Italy, we come to realise that we are being exposed to the darkness, or the "underground" emotions and feelings of the narrator. The setting therefore is a wholly appropriate place for the narrator to gain his terrible revenge - sealing a man into a room and leaving him to die a slow and terrible death. As we venture down into the catacombs, we go on a journey into the darkness and horror of the narrator's innermost desires, and thus we are shocked and terrified just as Fortunato is by what is revealed. Consider the following passage:

No answer still. I thrust a torch through the remaining aperture and let it fall within. There came forth in return only a jingling of the bells. My heart grew sick - on account of the dampness of the catacombs.

The way in which the narrator describes the completion of his task in such a matter-of-fact way makes his actions all the more chilling, as does his disavowal that the "sickness" of his heart had anything to do with his act.

Hope this helps give you an idea of the story as a whole and comments that you can seize on and develop in your work. It is a great story - vintage Poe, and worthy of enjoyment!