Michelle,
I trust you are going riding at a stable whose business it is to take beginners on a ride - if so they know what they are doing so relax and enjoy. A few tips - sit fully into the saddle, take a deep breath, sit up tall and just let your legs hang softly into the stirrups. Don't pinch with your thighs or your knees. The softer your muscles the more you will move with the horse and the less sore you might be when the ride is over.
Have a great time - let me know how it goes.