That was a couple of weeks ago.
For a tandem, the only advice to give is to relax and enjoy the ride. It's a once in a life time experience for most... don't ruin it by being nervous and freaked out the entire time.
ETA: dress comfortable for the ground conditions. You lose about 3 degrees for every one thousand feet,so it's going to be cold no matter what when you get to the top; but you are only there for a few minutes and you will have a jumpsuit on over what you were wearing on the ground.