Riding a Bicycle
From what I have seen, very little needs to be done to teach a child how to ride a bicycle. If children are given the opportunity and the right circumstances, they will work it out with very little adult intervention.
Children as young as two can use a balance bike. A balance bike is simply a bicycle with no pedals. Instead of pedaling, the child scoots along with their feet on the ground.
All you need to do as adult is provide the bike, help the child put on a helmet, and let them play. It can be helpful to a child to have a place with a slight downward incline in addition to a flat area to ride. At some point, the adult could mention to the child that if they want to stop or slow down, the child should squeeze the brake. The child will then probably spend a whole afternoon going then stopping, going then stopping, delighted with the newly learned feature.
The child should be ready to transition to a pedaled bike after a year or two.
Under no circumstances should you give your child training wheels! Just skip them entirely, they are a waste of time. They do nothing to teach a child how to really ride a bike.
There are many different types of these available on the market. They can cost anywhere from $40 to $200.
If you are starting with an older child who is too big for one of these, you can remove the pedals from a regular bicycle, though the child will have to take extra care not to scrape their legs on the metal crankarm to which the pedals are normally attached.