Congratulations on getting an interview! Keep reminding yourself - they think you're worth interviewing = you're potentially worth hiring!
Smile - at everyone! Obviously not in a 'crazy clown' kind of way - just in a friendly 'I'd be a great person to have around' kind of way - I recently found out that the whole company (well, those that I came across while in the office) were 'involved' in my last interview - everyone was asked for feedback (Receptionist, Secretary, Administrators that I passed in an open plan office)...
Also:
Research the company (have a really good look at their website as a minimum - and refer to it during the interview if possible to show you're prepared)
Go 'suited and booted' - even if the office is casual dress. This is your one chance to make an impression and stand out from the rest.
Don't rush your answers (consider the question - ask for clarification if necessary)
No one word answers - always elaborate (but don't waffle!)
Have some prepared answers (that show you in a good light) up your sleeve for some of the more typical interview questions like - "where do you see yourself in X years time?" "How would your last/current employer describe you?" "Do you have any questions for us?"
Good luck and do let us know how you get on!