While enotes editors do not write theses or essays for students, we are glad to offer suggestions and advice on your efforts. However, since no topic has been put forth, you may wish to first decide on what your topic is going to be. Enotes provides examinations of theme, character, style, etc. that may be good choices for an essay.
Another approach that can be taken in analyzing a short story is that of a different perspective. For instance, you can interpret Updike's main character as reaching an existential crisis where he must make an effort to identify himself as an authentic person; that is, he must create his own existence as an individual apart from the sheep who push their carts to the counter. This approach can work easily for Updike's story after you have read about Existentialism and its basic premises. In the introduction, define those aspects of Existentialism that you wish to establish in the essay and write a thesis based upon them. These three points will be what you, then, support with passages, quotes, etc. from Updike's story.
If you can, you may also wish to compare (and contrast) Sammy with the youth in James Joyce's "Araby" if you have read this story, also as both characters have an idealistic and romantic notion of the girls whose sexual attraction for them is great. However, they realize at the end that the girl(s) is/are not interested in them at all and, as a result, they suffer from their delusions. A "motivator" for the essay can begin the introduction.
See the links below for other ideas and "help."