Begin the proposal by answering the following questions.
What is the general topic?
Within this general topic, what is the question I will be discussing, or the thesis that I will be defending?
What are the relevant papers on this topic (you will need to do some independent research here). What are the views defended in these papers? Finding the entry on your topic in The Stanford Encyclopedia of Philosophy is a good place to start looking for references.
Then you will:
Write your introduction.
Write a plan. It should have two or three sections not including the introduction and the conclusion. You title each section, and state what you intend to do in each section.