If you think this confirmation will affect the way you wish to live your life then respectfully explain to your parents that you refuse to go through with it because you don't believe in God, have no interesting in doing so and don't wish to take part in religious acts in the future.
If this is just a one off event that will have no bearing on the future then I'd just go through with it to make your family happy while remaining an infidel.
If this is just a one off event that will have no bearing on the future then I'd just go through with it to make your family happy while remaining an infidel.