Its fairly straight forward to find out whats in a competency based interview. They want to understand your skills/experience/attitude. Each question is designed to test a particular competency they are looking for. So I would advise looking up what other people have said about the interview (maybe on wikijob). Additionally, look up general competency interview questions. It will be along the lines of "can you tell me a time when you've had a conflict with your manager" or "can you tell me about when you've worked in a team".
Personally I would collate a page of questions and practice speaking the answers to them out loud. Its can be difficult to recall the answer you want to give in the moment, and having rehearsed what you want to say will make it much easier. There are still likely to be questions coming up that you have to think up on the spot. Speaking to a computer is quite unnatural - especially if its not a person on the other end and they are just recording you. Id suggest recording yourself before hand - check everything sounds ok, the background looks ok etc etc.