I need to give a presentation about James Joyce's "Araby." What is the most important information to include?

A great theme to include in your presentation on James Joyce's "Araby" is the topic of disillusionment. It's one of the most important things Joyce touches on in the story, and so it might be worth mentioning.

The narrator of Joyce's short story is a little boy who is enamored with two things: the exotic adventure promised by the bazaar called "Araby," and his crush on a girl in the neighborhood, a character only known as Mangan's sister (the fact that she is only identifiable through her relationship to her brother is another potential fascinating topic to focus on, as it provides the fodder for a great feminist critique, but that's perhaps adding too much to your plate). The narrator idealizes both of these things, building them up to monumental proportions. 

In the end, the little boy is disappointed. "Gazing up into the darkness" (36) of the nearly deserted bazaar, the boy says "I saw myself as a creature driven and derided by vanity; and my eyes burned with anguish and anger" (36). In making this statement, the boy illustrates his disillusionment with his prior obsessions; recognizing the harsh realities of the adult world, the boy abandons the idealizations of childhood. Thus, one of the most significant topics in the story is disillusionment.