When kids reach the age of three or four, they can start to learn how to ski by going to resorts that provide group lessons. Very young children may benefit more from one-on-one lessons, because they require more attention.
If you want to teach a child yourself, use a harness or a device that can connect your child's skis to help them slide. Even a hula hoop over their shoulders and under their armpits can assist.
Since children can grow up very fast, rent children’s ski gear instead of buying it. Renting ski gear for children until the end of puberty will prevent having to buy new gear every one or two years.