Hey, I had a suit on for my interveiw. Best thing is to keep calm, its not that bad!. Just remember not to mumble, keep a clear voice and always keep a smart position i.e no sitting with hands in pockets etc. Go in with a positive attitude and use your initative and you'll be fine
Just remember, they're there to help you. Go in feeling calm and confident, not cocky mind! Just try and gather as much info from the chap as you can... as it's you who is wanting to get a job, and they're the one's who's job it is to get you into what you want to do.
In the interveiw they ask questions on personal life as well as your fitness i.e pull ups, push ups and 1.5 mile run time. They also ask a bit about your places of responsibilty and confirm your application. In my interveiw the recruiting staff also asked a bit about the current army deployments although I do not know if they ask everyone this question.