Hey Josh_J,
They're not going to ask you out of this world questions,I promise because the point of them interviewing you is so that they have a better insight of you as a person but some of the things they could ask you are:
Why you want to study the subjects you've chosen
Why you chose that particular school
What your interests are outside of school
What you want to do after you finish school
Why that school should offer you a place etc.
So those were the kind of,general questions they'll ask you but one of the schools I was interviewed at was a CofE school and they were,like,what do you think it means to be Christian and how do you think you could spread the values of Christianity. They also asked me the differences between A level and GCSE Spanish so I would strongly recommend reading through that school's course guide for the subjects you've chosen before heading in to the interview.
I wish you the best of luck and if you need any more help,let me know!
Diksha x