Starting the viva could make you feel a minute or two of nervousness. Your examiners will know this might be the case, so will respond accordingly. Simple questions to get things started. Perhaps a presentation, requested in advance, to give you a good way to begin.
Starting the viva is not the first thing you will do on viva day. Consider how you could plan ahead to begin your day well. Decide in advance how you will get to your viva. Decide in advance what you will wear to remove a decision from that day.
Starting the viva is a challenge, but not one that is wholly unknown to you. It is not something for which you have no experience to prepare you. Reflect on what you know about the viva and what you have done that can help you.
Starting the viva is almost the end of your PhD. Remind yourself that you could not have got this far by being lucky or by being unskilled. You must be good by this stage of the journey.