Stay calm and relaxed. If your horse senses your afraid then it will affect your horse the same way. They will question every little thing because your too focused on something going wrong. Are any other horses going with you? That will probably help your horse be more comfortable knowing that there is another horse with them :)
Relax and use the first couple rides to get an idea of what things make your horse uncomfortable. I also suggest you stick to walk/trot at the beginning....the trail is a completely different world than the ring to a horse.
And most of all....stay safe and enjoy..it's a great feeling to be out and about on your horse.
I was in your shoes about 6 months ago. When I went out I had to remember to keep breathing, also don't hold your horse too much, it might make him nervous and joggy. Talk to your friend too, it'll help keep you relaxed. Just have fun and enjoy the lovely ride.
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.