Summarize:

A helmet is standard equipment and should be purchased before buying a bike. Ensure the helmet fits the child's head size correctly and that the chin strap is snug, but not too tight.Look at pictures of properly fit helmets to find a helmet that fits your child. You may consider buying a bell or reflectors for your child's bike. Other safety gear to consider includes knee pads, elbow pads, and wrist guards. Cheaper bikes tend to be made out of cheaper materials, which also tend to be heavy. Because children's bikes have a smaller wheel diameter, they are harder to pedal, so you want to choose a lightweight bike. Choose a bike frame made from metal alloys and not steel. Children under the age of 5 tend to lack the coordination and skill to work hand brakes and should use pedal brakes. Pedal breaks are activated when the child pedals backward. If you choose a bike with hand brakes, make sure they can easily be grasped and that your child knows how to use them, as it can be dangerous if your child does not know how to stop her bike. Gears will help your child navigate hills and different terrain. Using gears are for more advanced riders, so make sure your child has mastered basic skills such as hand braking, turning, and safety before graduating to more advanced stages. Many bikes are available for purchase from a garage sale or through an online marketplace. Children will outgrow their bikes, so consider purchasing a quality second-hand bike as opposed to a new bike.  Look at yard sales or thrift stores for used bikes. Make sure the second-hand bike is in good condition before purchasing it. Check the recall list in your area to see if the model you plan on buying has been recalled.
Buy a helmet. Choose additional safety equipment. Consider the bicycle frame. Choose a braking method. Consider whether to buy a bike with gears. Consider purchasing a bike second-hand.