This is a very open-ended and personal decision on your part; writing an interpretation essay on a short story will depend entirely on your interpretation of the piece. In order to determine that, try asking yourself several questions. What did I get out of this story? What do I think that the author is trying to say? What is the main point of the story and how does the author make that point? If you think of some responses to that question, then that will lead you towards an interpretation statement that youc an use for your thesis.
I have provided several links below on writing essays, and on the story's theme and meaning, that might lead you in the right direction. Take a look at those, as they will be useful. In the meantime, to get you started, consider the following possible interpretations: In Raymond Carver's "Cathedral," he is making the point that allowing yourself to put yourself into someone else's shoes is one of the best things that you can do. Or, Raymond Carver, in his story "Cathedral," points out that empathy and perspective are the only way to truly experience profound emotion. Or, In "Cathedral" by Raymond Carver, the main character discovers that to let go of his own rigid perceptions of the world is the most rewarding choice he can make. So, those are some possible interpretations of the story that might work for a thesis statement. Then, go through the story and find key quotes and passages that relate to that statement, and you'll be set to go! I hope that helped a bit; good luck!