I believe that in terms of Updike's short story, "A&P," your connection is a valid one: in fact, when Sammy stands up for the girls in the A&P, he is not trying to impress them, for they have already left. Sammy has come to a place in his life, where he cannot remain silent when he sees something taking place that he believes is unfair. When the manager warns Sammy to think about what he is doing, he doesn't need to: Sammy has already made up his mind. He notes that on that night he has turned a corner and will never the same. The story is generally considered a rite-of-passage story, where a character takes decisive step to a place from which he or she can never return.

If you are an advocate, you probably didn't start being one just because it came with the job description. If I were you, I would either write about a time when you were coming of age (between 13 and 15), when you took a stand on something that changed your life. Or, you might write about something you are allowed to reflect on in a paper (that isn't covered under legal guidelines of your employer) where you had to make a hard choice. I think the important thing to remember is that when "Queenie" and the girls enter the store, Sammy is just another kid drooling over this beautiful young woman. However, by the time he leaves the store, he has taken a decisive step that not only shows that he is growing up, but most likely demonstrates the kind of man he will be some day—one who defends the rights, well-being, etc., of others.

Compare and contrast your experience to that of Sammy. Make sure that you cite specific examples from the story that are similar to (and different than) your own, in order to support the specifics of the essay. Since your instructor does not seem to have specified that you be Sammy's age in terms of your experience, I would think that anything that is distinctively similar between your life and Sammy's will fulfill the requirement of the paper, and then you simply need to point out the differences, as well.

For a thesis, I think I would start with something like:

Usually at some point in a person's life, he or she is faced with the decision of standing up for what he or she believes, or letting the moment pass. In John Updike's "A&P," Sammy decides that he must take a stand when he believes his manager is unfair in his treatment of young women who visit his store. A similar situation occurred in my life when…..

Make sure to provide an introductory paragraph with a thesis statement, as well as a conclusion. Stick to the same tense throughout the paper. Proofread several times. (You may already know all of this.) Good luck. I'm sure you'll do fine with wealth of experiences you have had in helping others.

Hope this is of some help.