Ride a Bike with Training Wheels
Many starter bikes for kids come equipped with training wheels, which fit onto the bicycle's back wheel and help the rider balance. With this method, the child learns how to pedal and steer first, without having to worry about keeping the bike upright. Once she has mastered pedaling and steering, you can gradually raise the training wheels higher off the ground so that the bike is a little more unstable.
As your child gains confidence on this less balanced bike, eventually she will be ready to have the stabilizers removed all together. Sometimes it helps to have her practice on a grassy area, instead of pavement, so falls are less painful. And of course, your child should always wear a bike helmet, whether she is on a tricycle or a two-wheeler.
Ride a Balance Bike First
This increasingly popular method takes the opposite tack: you teach your child to balance first, before he learns to pedal or steer. You need a special learning bike called a balance bike. Or you can start with a regular kid-sized bicycle, small enough (or with the seat lowered far enough) that your child's feet can easily touch the ground. Remove the pedals (temporarily) so he can focus only on balancing.
In this manner, your child will be able to control his own balance by putting his feet down on the ground when he feels unsteady. Start your lessons on a slight downhill, so your child gets the feeling of moving forward while balancing.
After your child is comfortable with coasting downhill, and pushing himself along on a flat surface, reattach the pedals and have him try putting his feet on them while he rolls down a gentle incline. Eventually, he will master turning the pedals with his feet; then you can try having him ride on flat ground. After a few practice sessions there, he may be ready for his seat to be raised to a more comfortable pedaling height, or to move up to a larger bike.
Ride While a Parent Pushes the Bike
If your child resists riding an undersized bike, or you don't have one available, you can try this old standby. Settle your child onto her bicycle, without training wheels, and hold her by the shoulders. Don't use the handlebars or seat; if you do, it subtly influences your child's balance and prevents her from learning the skill on her own.
As with the training-wheel method, use this strategy to help your child learn pedaling and steering without having to focus on balance just yet. Eventually, you can begin to let go and allow your child to balance on her own.
Tip: Don't let go without telling your child. If she falls at a time that she felt she would be safe, you lose her trust. Also, it's not safe for her to keep looking back over her shoulder to check on you.