Infants and toddlers start getting teeth from 0-12 months on average, the most common time is around 6 months of age. At this time, they aren't really eating solid foods and don't accumulate plaque. Once liquids besides breast milk and formula are introduced such as fruit juice, water should also be introduced to rinse the sugars away and balance the pH inside the child's mouth. Never let your child fall asleep with a bottle or sippy cup in his/her mouth. Once solids are introduced, a tooth brush should be introduced. At first it is OK to let the child get used to the tooth brush and just chew on it without tooth paste. As the child gets a little older from 1-2 years a pea size dab of tooth paste should be introduced, or tooth paste without fluoride. Parent supervision is always needed with fluoride at younger ages. Children should not swallow too much fluoride.
Supervision with teeth brushing is recommended until the child has good habits and dexterity. This usually happens in the early grade-school years as they are learning to write and draw. Without the dexterity to do a thorough job, tooth brushing may not be effective. There is a transitional period when children want to be independent, but still cannot to the job properly. At this point, it is good to let them first brush their teeth, then the parent can check to see if they did a good job, or it there is still Cheetos in the grooves of the teeth.
Once children are in late grade-school years, the habits should be developed and are hard to change. It is a good investment of your time to teach the habits in the early years.