Over-hand serve...??

I just made the seventh grade volleyball team and am ecstatic. Our first game is September 16, in just a few weeks. Unfortunately, it's required that all players serve overhand in games. I have a perfect underhand serve, it makes it over every time. But I suck at overhand. It always falls short or hits the net. I rarely make it over. Any tips on how to improve, FAST?! Thanks.