The answer to this question can be found in the first few paragraphs of the text itself. If you read it closely, you learn right off the bat that Montresor is angry with Fortunado because of the "thousands of insults" that Fortunado had sent his way. Because of this, Montresor has devised "revenge...with impunity." But, in order to exact his revenge, he must lure Fortunado down into the catacombs. This is much easier than it might seem. First of all, it is carnival, a huge party in the city, and Fortunado is already a bit tipsy from celebrating and drinking. Secondly, Montresor knows that Fortunado "prided himself on his connoisseur-ship in wine," and this was quite a big weakness for him. Fortunado thought he was an expert of wine, and whether one wine was better than another or not. So, Montresor devises a plan. He decides that he will pretend that he just bought a bottle of a fine wine, called Amontillado, and that he wants Fortunado to taste it and tell him if it really is a good wine, or if he paid too much for it. This of course would require that they go into the catacombs. Montresor knew that would appeal to Fortunado's sense of pride over his "expertise." Montresor is even sneaky enough to lead Fortunado into thinking that tasting the wine was his own idea. The conversation goes, in part, as follows:
"I was silly enough to pay the full Amontillado price without consulting you in the matter. You were not to be found, and I was fearful of losing a bargain...As you are engaged, I am on my way to Luchesi. If any one has a critical turn it is he. He will tell me—”
“Luchesi cannot tell Amontillado from Sherry.”
“And yet some fools will have it that his taste is a match for your own.”
“Come, let us go.”
“To your vaults.”
“My friend, no; I will not impose upon your good nature. I perceive you have an engagement. Luchesi—”
“I have no engagement;—come.”
So, Montresor is able to lure him down to the vaults on the pretense of having a cask of Amontillado that he was going to bring to someone else to taste to see if it was any good. Fortunado would not have that! HE was the best wine-taster, so HE must do it.
I hope that those thoughts helped a bit; good luck!