Quote From lindalou83:
In my experience, you will have already written a research proposal.
Actually in med sciences it's rare that the student has written a proposal - usually there is a specific project advertised.
Make sure that you can talk comfortably about any research experience you have. It's worth reading a couple of key references relating to the topic and getting a feel for what the key issues are. Do a little homework about the supervisors so you know what their research interests are.
Good luck.