If you are unsure how to answer a question, it's okay to say so. However, it is better to be honest, and admit your lack of knowledge than make up an answer that is likely to be wrong.
You need to take the interview seriously and show enthusiasm for the position. You should also research the company beforehand and prepare a few questions about their work culture or mission statement.
It is important to be confident and comfortable with oneself. One should be prepared for the interview by practising answers to common interview questions. You should also research the company and the position they are applying for before going into the interview.