I think you are off to a good start here. You have a topic -- the importance of Robert's sense of touch, considering he has lost his sense of sight. Now you need to think about what Carver is trying to say about the signficance of that fact. What theme is he trying to convey by having Robert's sense of touch be so important to how his character interacts with everyone else. Perhaps you could write about how when a person loses a sense, the other senses become hightened to make up for the loss. You could write about how touch replaces sight in Robert's ability to connect with other people. You could write about the irony of the story in that Robert, who can't literally see, actually has been insight into his own nature and that of those around him. This story is rich with details about the differences between the two men, and yet has such a powerful resolution with the narrator "showing" through the power of touch the catherdral idea to Robert, and through that process, learning to let go of his judgements and close-minded attitude and "see" the real Robert and himself from a new prespective. Your thesis must argue something that you can then prove with details and analysis from the story. Make a statement of interpretation of the signficance of the sense of touch and you should be on your way!