Simply have fun and keep your eyes and ears open. The first 10 or 15 minutes will be very easy as people get warmed up. Pretty soon the faster people will begin to gravitate to the front. Don't gravitate with them. Stay in the middle.
Which side you will pass or be passed on may depend on from which side the wind is blowing, just do as you see others do. If the ride develops into a pace line, you may eventually find yourself riding in front of everyone. Ride there for a moment, then fall off to the same side the rider who was last in front of you.
Make sure your tires are full, and that you have an extra tube. My first group ride ended early with a puncture.