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.
Good luck!