In order to write good literary thesis statements, you need to consider two things: what choices does an author make, and how do those choices contribute to the meaning of the work. Choices worth consideration in this novel include: the stream-of-consciousness narrator, by a third person narrator, who is of a younger generation of townspeople in comparison to Miss Emily; the five section structural divisions; the symbolism of things such as the house, her watch, and the sidewalks and house numbers, characterization of Miss Emily, her father, and Homer; the title of the story; selection of detail; and other figurative language.
Next you need to consider Faulkner's themes. The story is about love and unrequited love, decay and death, murder, the generation gap, the history of the South, loneliness and isolation.
In order to write the thesis statement you need to think about what Faulkner is trying to say about the theme. What is he saying about love? The story illustrates the extremes that someone may be driven to in the face of the "loveless" life that Miss Emily's father created for her by driving all of her potential suitors. What is Faulkner saying about the generation gap? Miss Emily represents the standards and attitudes of the old south, and her inability to accept the changes of the new generation, leaving her even more isolated than ever. Your essay would then discuss the specific literary choices that Faulkner makes to bring the theme to life.