The aim of the viva process is to satisfy the examiners that
You are not supposed, in other words, to know everything about your area of study, but only the aspects you investigated during your PhD research (check your university regulations!). Likewise, you are not expected to answer all questions perfectly, because no one can cover everything in depth in three years and, in any case, you are just at the beginning of your academic career and some flaws or gaps are still understandable.
On the other hand, you cannot look uncertain or evasive about the results of your research and the methods by which you obtained them: you must be ready and able to defend them by clarifying any queries from the examiners and at the same time support your choices with a convincing argument. In conclusion the viva is a demanding but not an insurmountable task, provided that you prepare for it. If you have not been given any documentation on viva-preparation by your institution, it is worth contacting your Graduate School or academic office to ask if they have any guidelines.