Your job will be twofold: teach basic skills (and rules), and teach teamwork - paying attention to what your teammates are doing during the game. Just have fun!
Work on the fundamentals of volleyball, setting, spiking, etc. Conditioning is key. Keep the girls interested and occupied and remember you want to keep them interested!
You should know the game of volleyball well enough to teach your athletes by description and also by demonstration. This will include how to bump, set and spike.