One of the most important aspects of a volley is the grip. The correct grip is the continental grip shown here. This grip allows you to hit the ball in a way that ensures it will go over the net.
The next important step in a volley is your footing and stance. When stepping into the ball for a forehand volley, step forward with the opposite foot like in the picture shown.
When making contact with the ball, it is important to not swing swing your racket. Just let the ball bounce off your racket. If you have a particular shot in mind, push the ball to make the shot.