The underhand serve is a good beginning serve. It does not provide a lot of power, but it can be very accurate and consistent. Volleyball rules require a toss of the ball prior to the serve. Coordination of the toss and contact of an underhand serve is actually quite challenging.
Think of the toss in the under- J hand serve as a release. Create a pendulum motion with your arms. As the hand holding the ball drops, your serving hand moves forward through the ball.
Contacting an underhand serve from a toss can be difficult. Keep the fingers of your serving hand pointing behind your body to expose the heel of your hand to the ball. This will allow you to contact the ball with the meaty part of your hand. Many players attempt to use a fist for an underhand serve, but this often causes inconsistent contact. In addition, the open hand leads to a more natural progression from the underhand serve to the overhand floater.