To train a horse, you've got to understand how they think. In the wild, horses are herd animals, with one stallion at the head of a herd of mares, but with a "lead mare" deciding where the herd goes.
Horses have a natural tendency to fear other animals, and people. When we approach a horse, it has no way of knowing what our actual intent is. It observes our actions and then makes a decision about what to do, flee or stay put.
Training has to be done a little bit at a time, getting the horse's attention, and then telling the horse what to do. You need the horse to see you as the lead mare. If you have a horse that is a natural leader, then you have a challenge on your hands.
Horses naturally look for a leader to tell them how things are. This is their natural herd instinct. It seems to work best when the human is the leader. The horse can follow its leader out of fear or respect. You will have a better experience with your horse if he follows your lead out of trust and respect. Formal training of a horse usually doesn't start until a horse is maybe 2 years old, but from the very start a foal is quite capable of learning the proper way to be around people. Spend as much time as possible with the young horse, so he can become accustomed to being around people.
When the horse is old enough to begin training, ground work is the first step before you ever try to ride him. This is called longeing. A longe line (pronounced lunge) is a long rope that you attach to the horses' halter. This gives the horse a large circle to move in while you teach him commands.
Lead training, teaching the horse to walk alongside you, and turning and stopping at your command, is mandatory. But watch out for a horse testing you by, for example, dropping his shoulder and leaning into you to get you out of his space. If you give in to this testing, he'll present you with many more difficulties.
The purpose of training is to teach the horse to allow a rider, and respond to the signals of the rider. You want the horse to respond when you ask it to do something, and not because you are constantly using a whip or crop to encourage the horse.