If you have a horse that has a shy nature, but hasn't been abused, it can also be difficult for the horse to trust you. All it takes is some time together. You will reconsider how you approach him. Make sure you move slowly and approach the horse from the left. If you are walking towards him with anything in your hands, stretch it out in front of you to show him there is nothing to be afraid of whatsoever.
Avoid wearing loose clothing when approaching a shy horse. The wind could pick up your shirt or pants and cause a sudden clothing flapping noise that will spook him. Once they associate you with any kind of fear, it will take much longer for the horse to trust you at all. The horse should never shy away from its owner over anything.
No one should push a horse to do something before he is totally confident. You will want your horse to trust and depend upon you completely before you ask him to jump or go down an unfamiliar terrain. You can instead have him build up his confidence by letting him jump over smaller hurdles and travel down simple, wide and open paths. If he jumps and stumbles, he might have trouble trusting you to jump again and you might second guess his decisions while jumping. So long as you know that each of you can trust the other, you will have a safe and successful ride.
If you have a horse that has been abused, it can be nearly impossible to gain the horses trust. You shouldn't give up hope, however. Plenty of horses have come back around after hours and hours of patience and love.
A good way to start with a horse that has been abused is to spend hours talking and sitting with him. Talk with a soft, calming voice and occasionally bring them a treat. Don't try to ride him and definitely don't ask him to do anything he doesn't want to do. It could take days before he is afraid at the very sight of you. Once he doesn't seem to mind your company, you can try to approach him. A frightened horse is a very dangerous animal, so be sure that you do not corner him. Offer him a treat and try to stroke his muzzle. If he eats the treat from your hands, you are making great strides!
If you can touch your horse and he lets you pat him, you are nearly there. You still cannot get on the back of a horse that has been badly abused without a professional trainer nearby who has years of experience with abused horses. You do not want to get on the back of a horse who will trust you on the ground, but utterly lose faith once you are on his back.