Dress modestly and professionally. If you were a boy we would all be telling you to wear a suit and tie or a blazer and khaki pants...You could go with slacks or a skirt suit--I am big wearer of pencil skirts and sweater sets and my DoD interviews tend to go very well--I would avoid jeans and sweatshirts or t-shirts with words on them--sort of the big 'women in the service rule' that no one talks about is "no cleavage"--this does not mean you cannot be feminine or ladylike and I can say it because I am a mom and I am in this business--it doesn't make me judgmental, it is a lesson that I learned when I returned to DC and started working in defense again--my big senior female mentor looked me in the eyes and said, "You are showing two inches too much cleavage right now. Go get a scarf or button your damn sweater. The guys will never say anything but they won't respect you." So, that is my advice to you as you embark on your first engagement with the military. (Not that anyone would think you were going to show up in go-go boots and a miniskirt or anything like that--just like my son would not think to show up in running shorts and sneakers with no socks.)
More importantly, be enthusiastic about learning to be a leader and a future Army officer. Go in prepared to talk about your major and what you migth want to do in the Army--although it is ok to go in and listen and say that you want to keep an open mind while you are going through training because there are so many opportunities in the Army. You can also ask about what the interviewer does in the Army--and how they got into it. You looked at the ROTC website for the school, right? Ask about the FTXs--it shows you looked at the website. Ask about Airborne School--because nobody wants to be a leg their whole life if they could be Airborne. Just be polite and look 'em in the eye. Also, I have found in interviews that it is beneficial to be upbeat--even if you end up talking about challenges or things that are bad--try to find the resilience that you took away from those lessons. And be yourself--but with less slang. Good luck.
One last little bit...be polite, respectful, and a good listener to EVERYONE you meet in that ROTC office...the ROO, the secretary/administrator...these people will become your family when you are in ROTC since you are interviewing at your university. You are going to know them and they are going to know all about you. Be extra kind. I don't think it will get you any extra points on the interview but you want your new family to like you and want to help you in the years to come. Leave them as excited to work with you as you are to work with them.
Ask questions about the program. Don't think you know all of the answers already. and Ask a somewhat personal question to the interviewer. like "What was your best experience..." "What do you like most about your years of experience" "What kind of advice can you offer me" Good luck