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!