Here is a Step by Step Process to Get Comfortable with Balancing on a Two Wheeled Bicycle.
At Cycle Path, we have found a method that seems to work for almost every first time rider (5 to 50). The first criteria is getting a bike on which the rider can touch the ground flat footed while sitting on the saddle.
Next, remove the pedals from the bike. Find a relatively flat smooth surface to ride on (ie: paved bike path) where there is little to no traffic. Let the rider scoot on the bike until he or she is comfortable, then find a slight and straight downhill and have the rider pick up his or her feet and coast down the hill.
Once the rider has become comfortable coasting, start raising the saddle in small increments and continue scooting to get comfortable balancing. The seat should be raised until the cyclist is able to sit on the seat and touch the ground with his or her tip toes.
When he or she is comfortable doing that, put the pedals back onto the bicycle. Now have the rider try to use the pedals.
Once pedaling stability has been established, make slight adjustments to the seat height so that there is a slight bend in the knee of the rider when his or her foot is at the bottom of the pedal stroke.