Well the first thing you need to do is when he/she invites you in scream at them that your getting the job and that's final. Interviewers and bosses will appreciate your gungho spirit.
Only kidding : ). In all seriousness wear something nice but casual. A nice pair of pants and a button down shirt for example. Just take a deep breathe and answer his/her questions calmly and be friendly. Nice to meet you *smile*. Stuff like that. Honestly whatever anyone tells you here is going to go flying out the window when your actually getting interviewed, I know from experience. It'll be over quick enough, just try to stay clear headed and listen to what they say.
Like Kuilui said, this is your first ever interview so when it comes to giving you tips on what to say and whatnot, you're going to be forgetting it the moment the first question hits.
I find not thinking about the interview and what questions may be asked helps a ton, problem is I begin to play off increasingly negative scenarios in my head and start stressing myself out when I do that, so just don't think about it and just let it happen, if I can put it that way. And of course, dress and smell well (Kuilui's recommendation is perfect) and just be a decent human being, you'll do just fine
I don't know if you've already gone for the interview but the most important thing is confidence.
If they've invited you for an interview, they already think you're qualified for the job, you just need to sell yourself. I find it easier to think of it more as a chat than an interrogation, they want to see what you'd be like to work with on a day-to-day basis and nobody wants to work with the nervous sweaty person.
I've done some seriously awful interviews in my time but this clicked for me a few years ago and I've been offered every job I've interviewed for since then. Just be pleasant and act like you really know your shit.
Also, vitally important, if they ask if you have any questions at the end of the interview, make sure that you have a question to ask. Preferably, ask something that would matter if you got the job, something about how they carry out certain tasks or what would be expected of you in a certain situation, do not ask about money. This shows your interest in the job and that you're already thinking about your potential duties as an employee.