Rachel,
The volleyball serve is more about technique than strength. If you put all of your body's forces together properly, the distance and direction will come. This involves a low controlled lift, a weight shift at the same time from the rear foot to the front foot, a ball contact with an open hand to the center of the ball, and a controlled follow through. Some younger players have mor success with a multi step lead in prior to the serve (2-3 steps). A little practice against a wall should help you a bunch.

You can also start your new technique at the 10 foot line and as you make it over 5 times take one step back. Keep doing this until you get yourself back to the serving line. This may take awhile so do not get upset if you don't get it on the first try. You have taken time to learn to punch the ball, now you will have to take time to use an opened hand. When you start getting it over you can start working on control. ( putting the ball where you want it )