I was interviewing for new positions not long ago, and one of the standout pieces of advice that I took with me was to treat the interview as a learning process. Go into it with the mentality that no matter what happens, you're gaining interview experience so that your next one - whether that be for a promotion, a new position, whatever - goes that much better. This personally really helped me calm down about whether or not I'd get the job, and as such I was able to give a much stronger interview than I would have had I been so worked up about making sure I "passed" the interview at all.
Previous answers about being genuine and asking good questions are also great advice, I personally always ask what the interviewers' favorite part about the company is, "what qualities would make somebody successful in this position?", etc.
Overall, just be yourself and don't be afraid to be honest - answering "I don't know" doesn't immediately write you out of the position so long as you're willing to learn as you go.
Good luck! The fact that you have an interview at all shows that they're interested, remember that and be confident in yourself.