When you write an essay proposal you need to think about the reason you are being asked to do so. Essentially, your teacher is giving you an opportunity to get some advice about the main ideas in your paper before you have committed substantial amounts of time to research and writing. Also, many teachers use this sort of proposal assignment to steer students away from unworkable topics.
The proposal should show that you have a clear and distinct thesis that is narrow enough to be covered thoroughly in the word count allotted for the final essay. You should also show evidence of having done some preliminary research, and include a bibliography of scholarly sources.