How do I prepare for the interview if I have never studied the subject I am applying to before?

First of all, keep in mind that you are far from being the only one in this situation. I had never studied social anthropology or international relations before my interviews in Cambridge. The examiners are not going to assess the amount of knowledge you already possess on the matter. Rather, they want to know how capable you are to make use of the knowledge you do have, in order to make a discussion interesting and fruitful.
For this reason, the questions you might be asked tend to cover broad topics, such as 'what is the relationship between climate change and war?'. Your job is show them how you navigate between those general concepts by speaking out loud the process of your thinking whilst you try to answer the question. A good way to do this is first explaining why you find the question compelling. If countries tend to wage war for access to resources, such as water, and if climate change involves a drastic reduction of those resources, does it mean that we are going to have more wars in the future? Then, it is always good to show that you are able to connect dots together, by introducing authors or books you have read into the conversation, even if they are only distantly related to the subject, and show how they can bring something to it. Again, it is important to bear in mind that the examiners have already invited you to an interview on the basis of your personal statement and your grades. They know that you are a good student. What they do not know yet is that you are an interesting and pleasant person to have a discussion with.