Welcome to the volleyball community! I'll offer a few more suggestions to add to the excellent list started by PARef.
- Don't listen to anyone who says that doing volleyball is easy. Like you, I came from another sport, softball. While the physical aspects are not as grueling, the mental aspects are very taxing. When I have a fellow umpire ask me about volleyball because they hear it's "easy money", I tell them it's easy money just like softball would be if all you had to call was balls and strikes or safes and outs.
- Start learning about the different volleyball offenses that teams will run. Life will be easier when you start recognizing where your setter for each team is coming from, and that begins with watching the teams warming up before the match even starts. Make it a point to watch the warmups even if you are just spectating, and that will help you understand its importance. Here is a good explanation of offenses that helped me: Volleyball Offense Explained and Detailed.
- Watch these training videos found at: USA Volleyball Referee Training & Education: Training Videos. These are USAV videos, and there will be some differences from working an NFHS match, but overall I think these are great for learning some basics about your responsibilities.
I hope you enjoy working volleyball. I certainly have. I think you will find the coaches to be much more civil than those you encounter working basketball!