How should I prepare for an Oxford philosophy interview (for PPE) if I have never studied philosophy before?

Making a good impression in your philosophy interview at Oxford will be necessary to earn a place to study PPE. Oxford philosophy tutors will not expect you to have studied philosophy before, and they will not expect you to know any specific philosopher's work in detail. Tutors are mainly interested in seeing if you can think on your feet in a clear and methodical way.
Philosophers deploy arguments to advance or critique an idea or a thesis. An argument is a set of premises followed by a conclusion. Understanding the different varieties of arguments will help you to understand and engage with philosophical concepts. If you can identify what variety of argument you are dealing with, you will be better able to see if the argument is successful or faulty. Identifying an argument as deductive or inductive is an important first step in offering appraisal or criticism.
Examples:
A deductive argument is one whose conclusion is true if all the premises are true (the truth of the premises guarantees the truth of the conclusion). For example:
1 - Mary is taller than Joe 2 - Joe is taller than Smith therefore: 3 - Mary is taller than Smith (conclusion)
If Mary (180cm) is taller than Joe (179cm), and Joe is taller than Smith (178cm), then it necessarily follows that Mary is taller than Smith.
An inductive argument is one whose conclusion may be false, even if all the premises are true (the truth of the premises does not guarantee the truth of the conclusion). For example:
a) - Mary has been to watch every Liverpool game b) - Liverpool are playing this Sunday therefore: c) - Mary will watch Liverpool play this sunday (conclusion)
Whilst it may be true that Mary has been to watch every Liverpool game, and Liverpool are playing this Sunday, Mary might not be able to go to the match on this occasion.