If you have a young horse, or an older one that does not have trail experience you want to start them trail riding the right way, but giving them a good experience so they can learn to relax and enjoy being out on the trail.
I start by leading the horse on a few short walk around the farm and neighboring fields. This allows you to see how much your horse perks up out of familiar surroundings, as well as teaching him that you are still in control. Ideally his first ride out will be with a more experienced trail horse. This way your horse can see there is nothing to fear, and if he does spook or balk at something, the more experienced horse can walk past him and over or by the spooky object. We use this same technique starting horses over cross country fences. The green horse trots about a horse length behind the older one over the first small fences and ditches so that he can just follow along and does not learn to stop and refuse. If the green one does stop at a jump or continues to refuse to walk past something, bring the experienced horse back and walk past again. This method allows you to get the new horse to get used to everything without starting a fight by having to kick like crazy!
If you do not have a good lead horse, then just slowly build up the distance and duration that you are out on the trail. Start with just a short walk after a ride in the arena and build on that. With a little patience and consistency, you can have a good trail horse in no time!