The best way to write an essay that analyzes John Updike's "A&P" is to first decide what aspect of the story you want to address. This is a often anthologized story, and very often its "rite of passage" theme garners a great deal of attention.
Writing an essay about Updike's short story, I would concentrate on how Sammy, our narrator, changes from the beginning of the story until the end of the story. This is where the "rite of passage" element presents itself. Before the girls walk into the "Sammy's" store, he is just a kid, working part-time, admiring an exceptionally regal and beautiful girl in the store.
A good deal of the story describes Sammy's typical behavior: how he notices "Queenie," admires every detail of her appearance and her behavior, and finds himself defending her. The pivotal point of the story is when Sammy's manager scolds the girls for entering the store, defying the store's dress code. Sammy decides that he must speak out when something happens with which he does not agree. He steps "into the ring" as a person who fights for causes—even the hopeless ones—because he believes he is doing the just thing. Sammy tells Mr. Lengel that he unnecessarily embarrassed the girls, and then Sammy quits his job. His boss knows he'll be sorry for walking out on the job:
You'll feel this the rest of your life...
Sammy knows this, too, but he also realizes that he has stepped over a line in his life and there is no turning back—he's taken a step into manhood.
...my stomach kind of fell as I felt how hard the world was going to be to me hereafter.
Ironically, Sammy has done something that actually speaks to his good character.
Once you have an idea about your topic, you'll need a thesis statement in your introductory paragraph. Here you introduce the topic you will write about. There are a variety of ways to do this, but for me, I would probably write something like...
Some young men go through a rite of passage by going to war; some study to pass a religious "test;" and, still others notice that within a moment, one's perception of the world changes forever. Sammy, in John Updike's "A&P," does not raise a weapon, but the change he goes through alters the way he looks at himself and his place in the world, forever.
In this introduction, I have mentioned what my topic will be about. It identifies the concept of "rite of passage," and gives some general examples (not from the book). It then identifies the story and author, and what Sammy does not go through, but how his life changes—today!
The second paragraph should be the first BODY paragraph. This is where you give examples of how he's just another kid. You make the general statement and then provide examples from the text.
The second BODY paragraph shows how Sammy has changed once "Queenie" was scolded by the store manager; your should provide specifics to support this element.
The THIRD body paragraph can compare and contrast the differences, and may even predict how Sammy may act from now on.
The CONCLUSION (which is the fourth body paragraph, but the FIFTH essay paragraph) redefines the topic presented in the introduction, but does not repeat details or introduce new information. You "pass judgment" on the topic:
Everyone goes through some "rite of passage," either a socially accepted life-changing event, or by virtue of an "epiphany"—an eye-opening experience that, as it does with Sammy, alters how one views the world and self.