Montresor let Fortunato know about him buying a cask of Amontillado because he knew that the latter had a weakness for wine. Fortunato had great pride in his connoisseurship in wine, plus he was a drunkard. Montresor vowed to get revenge from Fortunato for his "one thousand insults." He made up a story about the Amontillado and lured Fortunato to check if it was real (by playing on the latter's vanity) for him to realize his revenge. He knew that Fortunato wouldn't pass up a chance to taste the Amontillado, trick Montresor into believing it was not actual Amontillado, and then purchase some of the expensive fine wine for himself at a cheap price.
This plan was illustrated in the first three paragraphs of the story.