Carefully. Until children are about 5 years old parents should be brushing their teeth. This ensures a good cleaning and no missed spots. Use about a pea sized amount of toothpaste on their toothbrush and brush away. Once your child has a tooth you should start brushing (at first you can just use a warm washcloth to get your child used to the motion). Once two teeth are together time to floss!
Ask dentist. 1st dental check-up age 1, preferably w pediatric dental specialist. Ask dentist to show you how to clean teeth, what type of toothpaste, if any, to use (depending on child's age), how to manage fluorides, check crania-facial growth and development, review diet (harmful foods), how to clean between teeth, etc. Go to dentist for hands-on instructions.
Soft toothbrush. Just purchase a small soft toothbrush and gently brush them in the direction that the teeth grow.