This is a common problem for athletes. I would say that an athlete is never too young to start working on their mental game and setting goals (individual and team goals). Over the years, I've found talking about the mental game as an athlete and a coach helpful in taking away some of the nerves.
Applying visualization techniques can be good insight into understanding what is going on inside players' heads. You can have everybody sit down before or after practice and go over their pre-game/during the game/ post-game thoughts. It's important to think positively about how we plan on playing before the game. Having a plan is also helpful because we feel prepared and ready. It's kind of like when you study really hard for an exam, you are less nervous because you are prepared and confident you will pass the exam. A volleyball game is very similar, when we feel prepared mentally and physically, we are less nervous and more confident in how we play our game. How we want to feel going into the game, how we want to attack the ball and communicate with one another on and off the court are all things to consider. Tell you players the night before the game, before they go to bed to visualize themselves in certain situations and being successful in these situations. For example, pretend it's the game point and the ball is passed to the back row and the back row passes to the setter who then sets the ball to you and you call off your teammate to make an excellent kill, which you've been working on all week in practice!
Try to remember that there are many things, we as athletes and coaches cannot control during a game, such as the number of kills we accumulate or the way the referee calls the game or our win-loss record. Don't worry about what you can't control, worry about what you are able to control. If a player makes a couple of mistakes during the game, tell them to let it go and leave the analyzing for after the game. If you see one of your players spinning out of control or so nervous they are not focusing, pause a minute and ask them to take a deep breath and clear their mind. A couple of deep breaths can do WONDERS, sometimes we just need to be reminded to breathe!
I could go on and on, but I'll leave it at that for now. =)
There are actually many books written about the mental game of sports. Some of my favorites are Heads-Up Baseball and Mind Gym.
I think It is most important to remind players that although they are playing to win, it is supposed to be fun too!