There are many possible angles to take in writing about John Updike's story "A & P." It all just depends on your opinion of the story and whether any particular aspect of it stands out in your assessment as being worthy of note. A couple of ideas that one might explore in a thesis have to do with Sammy's reaction to the girls; Sammy's moral predicament; Sammy's choices in his moral predicament; the theme of individuality; or Updike's authorial style choices.
One could take a position in relation to Sammy's reaction to the girls' attire as being representative of Marxist literary theory with Sammy standing for the exploiting capitalist class and the girls standing for the exploited worker class. One could discuss Sammy's moral predicament while listening to the manager scold the girls in terms of a rigid archaic Victorian social moral code that not only constrains but also ostracizes. One could discuss Sammy's choice to quit his job in terms of the irrationality and shortsightedness of youth that acts as self-sabotage and causes endless difficulties.
One could discuss the self-imposed constraints of an individuality that doesn't leave room for social realities, or, on the other hand, that would enlighten society if a demonstration of individuality could initiate a successful rebellion against status quo. One could discuss Updike's choices in vocabulary, discussing his use of adjectives and adverbs or his choice of prepositional relationship words.