Focus On Your Feet Not Your Arm
Many players playing their shots focus on their arm hitting the ball. When it comes to volleys there is no difference, especially when players are at the net.
The arm action should be a short, compact but fluid motion with almost no backswing at all. Concentrate on shortening your swing. The shot uses the speed of the incoming ball to generate pace, then use your legs by stepping forward as you make contact with the ball.
Some volleys this is not possible, sometimes close to the net the volley is played as a block volley. Ideally the forward motion of the racket comes with footwork and moving forward.
Keep Your Racket Up High After Contact
A good volley sees the racket moving forwards on rather a straight line. Most junior players tend to force the racket downwards too soon.
Even if the ball is above net height you should not be tempted to hit down but rather straight, let gravity do its job.
When practicing the shot look at your racket position at the end of hitting the ball, see if it is too low and try to adjust this.
Let The Ball Come Onto The Strings
As already mentioned, many players tend to swing too much at the volley. If you are using too much arm action it makes it quite impossible to have control on your volleys when the ball is coming fast at you.
A good tip to try and eradicate this is to let the ball come onto the strings of the racket, rather than moving the racket to the ball. This will help to shorten your swing and make you hit the ball cleaner.
This does not mean that there is no arm action whatsoever, the tip is to try and get you to shorten your swing and make you aware of how you are playing the shot.
Volleys are a great way of establishing a winning shot. They help you to be attacking in your play and give you confidence of moving to the net early. Practice hard with your tennis coach, and develop this part of your game as you will reap the benefits in the long term.