If there is one thing that has to be talked about in the viva it is the significant, original contribution that you’ve made through your research and that you have written up in your thesis. It’s not the only thing that can or will be talked about but it has to be discussed. You have to talk about it because this research and thesis is what you’ve done to earn your PhD.
So you have to be ready to do that. Write summaries about what you’ve done, make notes, check your thesis, have a mock viva or in some way practise with questions talking about it.
There will be lots to talk about at your viva. Talking about your significant, original contribution is guaranteed. Practise doesn’t make perfect, but prepare and you will be ready.