Understanding what your viva will entail and preparing for that will initially start with discussions between yourself and your supervisory team, identifying what areas you will need to focus on e.g. academic questioning.
Your supervisory team will have experience of the examination process and will then be able to work closely with you by holding mock viva scenarios and generally supporting you in your development.
The Vitae website also has guidance for Research Students on the preparation for your viva. The below link should be a good starting point:https://www.vitae.ac.uk/doing-research/doing-a-doctorate/completing-your-doctorate