Amy
You might want to have a look at the following video, it was made by the Ministry of Justice and is an ESA appeal so there will be some difference, for example there are three members to a PIP panel, but it will give you some idea of what happens.
MOJ Video of ESA Appeals Process and Tribunal II would familiarise yourself with the contents of your submission, but beyond that, just try an relax. For my first appeal I spent the two days before, working out a master plan of how I was going to convince the panel of how I met the criteria, it lasted about two minutes and I wish I had spent the time watching a good film.
Best of luck
Gordon