The University of Virginia health system recommends that you begin cleaning your baby's teeth with a soft, wet cloth every day once the first tooth appears. Once the baby is about a year old, you can begin brushing his teeth with a small, soft toothbrush and water (not toothpaste). Around a year or a little sooner is also a good time to take your child to the dentist for the first time. Once your dentist sees your child's teeth and how many have emerged, he or she can advise you about when your child should begin using toothpaste, and what type of toothpaste it should be. He will start by using a dab of toothpaste about the size of a grain of rice.
Another important thing to know about caring for your child's baby teeth: do not leave your baby in bed with a bottle unless the bottle contains only water. If the baby drinks milk or juice and then falls asleep, the sugars that are still on his teeth can cause his teeth to decay. He needs those baby teeth in order to chew solid food, learn to make certain sounds in speaking, and save space in his mouth for his permanent teeth. If you keep his teeth clean and avoid sending him to sleep with milk or juice on his teeth, his baby teeth should stay healthy and be able to do all three of these things.
Resources:
http://www.nlm.nih.gov/medlineplus/ency/article/002045.htm
http://www.thenewparentsguide.com/baby-teething.htm
http://www.healthsystem.virginia.edu/uvahealth/peds_dental/teething.cfm
http://familydoctor.org/834.xml