A horse doesn't automatically get born, become tame and be suitable to ride. It needs to be educated or trained, and as with any animal, the earlier the training begins, the easier it is. The following information will give you an idea of what is involved.
Getting used to handling
A foal should ideally be handled as soon as possible after birth so that it becomes comfortable with humans right from early in its life. Place a mare and foal in a small yard (don't separate them), and approach the foal slowly. Be calm and pat the foal gently. Lift the legs slowly and place your hand on different parts of the body gently. Spend no more than a few minutes doing this. Repeat the process frequently in the foal's early days. (If you leave these lessons for several months, the job will need to be handled differently as the animal will be much stronger).
Learning to be led
The foal must next be taught to be led in a headstall. The foal and mother should be moved into a restricted area, perhaps using a crush to separate them. The foal should be allowed to relax before putting any head collar on it. When relaxed it will sigh and chew: if tense it will hold it's breath and clench the teeth. It's normally best to secure the neck strap first, then put on a nose band. Place this gear on very gently. Once the head collar is attached, the mother can be led out. The foal will follow, being led by a handler. When it feels the collar the foal will create a fuss. The handler must hold the rope, but not restrict the foal too much. It usually only takes a matter of minutes for the foal to get used to being led. The process is repeated every day for at least 3 days; for 10 minutes at a time. Gradually the foal is taught to be led away from the mother
Other lessons and procedures
Foals must be taught a variety of other things including to be tied up, to have their legs handled and to eat solid food (weaning). As the foal develops other procedures may be necessary including castration, branding, vaccination (eg. tetanus and strangulation) and keeping official records. As the horse continues to grow the training will continue with the horse being taught:
Breaking in a horse is an "expert" job and must be done properly. It requires an experienced handler, the right equipment, short effective lessons and a sensitive but firm attitude.