"The Necklace" is a story whose most prominent element is theme; specifically, the theme of materialism. This, then can be the topic, and to arrive at a thesis ask, "So what about materialism?"
One answer is that Madame Loisel cannot be happy in her marriage as she values material things more than spiritual ideals. For, Madame Loisel's belief that beautiful things and luxury are essential to her happiness is the fallacy that leads to the ruin of her happiness, comfort, and her physical beauty. (This equals a thesis statement.)
The five-paragraph essay contains a thesis that has three opinions in it. So, the three opinion parts of the thesis are (1) ruin of happiness, (2) loss of comfort, and (3) destruction of her appearance. Forming topic sentences out of these three opinion concepts will start off each of the body paragraphs. For example, the topic sentence for the first paragraph can be something like this:
Madame Loisel is always discontent with her social situation and does not appreciate her husband's love (supported by the text quotation):
She grieved incessantly, feeling that she had been born for all the little niceties and luxuries of living.
Mme. Loisel looks at her home and what is in it as inadequate, feeling she has been born for nicer things. She does not appreciate what she has, [give examples] nor does she appreciate the affection her husband holds for her [give examples]. For instance, he is willing to postpone his purchase of a rifle in order to provide her money for a new dress for the ball to which they are invited. But, this is not enough; she feels that she needs a necklace for the dress....
Another idea that can be worked into an essay is how Madame Loisel's false pride leads her down a path in which, as Robert Frost writes in "A Road Not Taken," "way leads to way." Her plan of deception that she is a charming lady at the ball, one who has material possessions, leads to her demise. For, her false pride does not permit her to apologize to her friend, Mme. Forestier, who has lent her the necklace, and tell her that she has lost it. Instead, she assumes that the necklace is made of diamonds, and she mortgages everything, impoverishing herself and her husband because of her false pride in order to replace it. She holds this false pride to the end, as years later she boasts to Mme. Forestier that she has paid for this necklace now after many years only to learn that the original was a mere imitation.