Prior to the eruption of your child's first tooth, gently clean his or her gums after bottle or breastfeeding with a soft, wet washcloth. Continue in this fashion until the first tooth actually erupts.
After the eruption of the first tooth, brush the new tooth/teeth with a soft, infant-sized toothbrush two times a day. Use a very small amount of fluoridated toothpaste for children under two years of age (the American Association of Pediatric Dentists [AAPD] recommends a rice-sized amount) as it is impossible to keep infants from swallowing the toothpaste. If your baby is uncooperative with this process, which is to be expected, laying the child on their back on either the floor or the bed may help.
The AAPD recommends a pea-sized amount of fluoridated toothpaste for children from two-to-five years of age. You should continue to brush your child's teeth for them until they are around the age of eight.