I'm trying to write a thesis statment for my research paper... about the symbols of "The Metamorphosis".
Concerning your need to write a thesis about symbolism in Kafka's "The Metamorphosis," let me give you an idea of what you have to consider. The enotes Study Guide on the story says:
The haunting story of a man transformed into an insect has attracted numerous commentators, who while agreeing on the high quality and importance of the story, disagree strongly about what it means. Freudian, Marxist, existentialist, and religious interpretations have all been proposed, and there has been debate over whether Gregor Samsa, the man-turned-insect, symbolizes the human condition.
It is generally agreed, however, that the story portrays a world that is hostile and perhaps absurd and that major themes in the story include father-son antagonism (perhaps reflecting Kafka's difficult relationship with his own father), alienation at work, isolation, and self-sacrifice.
The story is sometimes praised for its symmetrical, three-part structure and its use of black humor, and its symbols (such as the lady in furs and the music played by Gregor's sister) are sometimes puzzled over, but what makes the story memorable is the central situation of the transformation of a man into an insect and the image of the man-insect lying on his back helplessly waving his little insect legs in the air.
You'll need to decide what you believe the story means before you decide what symbols you want to write about. Then you'll have to figure out what the symbols are symbolic of, as well as what they mean.
If we get more specific, enotes also suggests this essay topic that centers on the woman wrapped in furs:
The picture of the woman wrapped in furs on Gregor’s wall is something that he refuses to part with. Write an essay showing the importance of this picture in Gregor’s life and, if possible, try to explain its symbolic meaning.
That's one possible angle about one possible symbol. There are numerous symbols and angles. Decide on those and your thesis will come more easily.
Of course, the dominant and most important symbol is Gregor himself. Decide on the meaning of that symbol and you'll have taken a giant step toward interpreting the story.