I believe that many students cause themselves problems and even writer's block by trying to come up with a thesis statement before they have written their paper. A thesis statement comes first in a paper--but it doesn't have to be written first. And anyway, your first attempt at formulating a thesis statement does not have to be the final one. You should just say the words, as E. B. White once advised. Don't try to make it sound graceful or pretty. Just try to get your general idea down in words, regardless of how many words it takes. Then ask yourself if that's what you really mean. And if it isn't, rewrite it. The best writing is rewriting. If you haven't written your paper yet but are just stuck on the thesis statement, forget about the thesis statement and write the paper--or write as much of the paper as you can. Your thesis statement might be hidden in your paper.
You say in your question that "The Garden Party" is about the relationship between rich and poor. What is that relationship? That seems to be what your paper is about, and what your thesis statement is about. It occurs to me that the relationship between the rich and poor, the servants and the guests, is observed and understood through the point of view of one young girl. (It has been a long time since I read that story, and I don't remember it very well.) So "The Garden Party" is a story about the (????) relationship that exists between the rich and poor, as seen through the eyes of young Laura. According to the enotes Summary:
It leads both Laura and the reader to an epiphany—an enigmatic moment of revelation that, in this story, is comic and overwhelming at the same time.
It seems to me that that "epiphany" ought to be the most important part of your thesis statement. The Summary states that it leads both Laura and the reader to an epiphany. You were the reader. What sort of epiphany did you experience? Has it changed you at all? Is there really any "relationship"? Or do the privileged guest just take the servants for granted, as if they were not really humans but sort of robots?
I think the key is in that "epiphany" because that is what the story is leading up to and what the story is all about.
Writing thesis statements is hard. I remember turning in several papers in high school with teacher comments like "your thesis is weak" or "you don't have a thesis." It was never fun. That is until I figured out that you can write a good thesis if you follow a formula. I like formulas. You plug in x and y and get z.
I like to use two different thesis writing formulas.
Formula 1: "Make bold statement and support with three reasons" formula.
Formula 2: "The although and counter argument formula." I like this one more because it forces you, the writer, to admit counter arguments and rebut them.
I'm not allowed to write out an exact thesis for you using your piece, but I'll give you a few examples and try to get you going in the right direction.
Bold statement thesis example: "Macbeth deserves all of the blame for Duncan's murder because he plotted doing it, actually did the murder, and continued killing afterward."
"The Titanic was doomed to sink because its hull was made from inferior steel, lacked proper bulkhead doors, and had a captain that put speed above safety."
Both of those thesis statements make a claim and then support it with three specific reasons. The body of the paper will attempt to prove the statement by going in to depth on each of the mentioned topics.
You said that you want to focus on the rich/poor elements in "The Garden Party." Maybe start off by saying something like "The Garden Party' shows a clear polarity between the rich and the poor through "x," "y," and "z." You will have to fill in your reasons.
The although and counter argument thesis example: "Although Romeo appears to be deeply in love with Juliet, he is likely more in love with the idea of being in love." This allows you to write about how Romeo can be considered in love with Juliet. But then it also forces you to put down those arguments with your main case.
"Although DNA fingerprinting has led to much stronger court cases, the need for thorough investigative detective work is greater than ever."
For "The Garden Party," a thesis using this formula could be: "Although by the end of the story Laura seems to understand what wealth and class distinctions look like, it's clear that Laura still . . . "
I hope this helps and good luck!