Talk lots, sing, breathe!!
Be considerate of your horse. If it's rocky, take it nice and slow and let him steer through it. If you're going uphill, lean forward just a smidge to help him up. Down the hill, make sure to keep him nice and slow. Loose rein on both occasions. If he starts getting too fast, gently remind him that he's here to relax. Massaging the horse's neck usually helps most horses to relax.
Push him through if he hesitates. But don't do anything huge on the first trail ride like wade through a stream/lake or go up some serious rocks. Take it nice and slow.
Have fun :P And be prepared for animals popping out like squirrels or deer.
"Strength is the ability to use a muscle without tension"