I agree with the above poster as to structure for this essay. A few additional comments that you might find helpful with regard to choosing character traits:
Look at each character as an individual - a real person. What kind of person ios Tom, for example? He is a frustrated writer who works a menial job to support his family. He is a caretaker who wants to escape. He is a gay man living in a heterosexual world. Traits that he evidences include caring and concern for Laura and his mother, and dissatisfaction with his life's circumstances but a sense of conflict within himself over his own desires.
Amanda, on the other hand, is a dreamer, just like Laura. However, Amanda's dreams are tied to a past that no longer exists. The southern gentility that Amanda wants to still exist in the world has been replaced by an Urban society that no longer values afternoon teas and gentleman callers. She is a single mother who is always trying to keep up appearances and do what is best for her family (selling magazine subscriptions to pay the bills, encouraging Laura to learn a trade) but she is often stifled by her inability to see reality ar, more to the point, to accept that reality.
Laura is insecure. She hides in her glass menagerie because she sees herself as different and inferior. It is her inferiority complex, more than her physical handicap, that imprisons her.
Jim is the opposite. He is overly confident, a believer in his dreams, but, unlike Amanda, he is willing to chase after those dreams fully believing that he will realize them. He does not allow obstacles to linger in his path.
The first task in answering this essay prompt is to write the thesis statement: Which character have you chosen? Which character trait will you focus on? Then you should focus on finding lines either from the character him/herself or from other characters to prove that trait describes the character. Devote each body paragraph in your essay to explaining how each line proves how the trait matches the character.
For example: paragraph 1 should include your thesis and a general introduction to your essay; paragraphs 2, 3, and 4 will each have a quote from the character that demonstrates how the character fits the trait, paragraph 5 will re-state your thesis and conclude your essay.
This is just one way to approach your prompt, but will get you a good grade. The trick is choosing the trait to describe the character and then choose the quotes that demonstrate that trait. Your teacher wants to know that you understand characterization and can apply that knowledge when you write the essay.
I hope that helps! Feel free to post again when you decide on your character and trait; I'll check back to see if you need more help!