Be sure to bring your own tools and uniform, if possible. Also, bring a small notebook and a pen. They will most likely give you a tour and start you off with fairly simple prep tasks. Ask for a demo to be sure you know exactly what they want. Until they know your skill level, they are not likely to give you anything difficult to do.
Try anything they offer you to taste.
Behave professionally with the line cooks. I know that seems basic, but as a cook I had several stages confide things in me that they may not have wanted the chef to know (such as their endless complaints about their current or previous chefs). As soon as they left, the chef would ask me what I thought of the stage, so keep this in mind.
I was always nervous for the stages I did, too, but they all ended up being good experiences. Good luck!