Katie,
If you get nervous in interviews then do prepare for it beforehand. Feeling prepared will help you relax knowing you will give it your best shot. I recommend "Airline Pilot Interviews" by Irv Jasinski as a good book to read - take some of it with a pinch of salt as it is very American but there is some very good stuff in it. Get someone to perform a mock interview on you and try and anticipate the type of questions you are likely to be asked.
Remember you are being interviewed with a view to ultimately flying a jet with 150 people on it - hardly a nerve free experience the 1st time believe me!