Kids should be introduced to a toothbrush once their back teeth appear. Before that use finger to clean after every meal/snack. Till 4 years of age, parents are advised to brush for their kids and till 8 years under supervision. This will ensure healthy teeth.
We started with both kids once they started getting teeth. You can get age appropriate tooth brushes and tooth paste. For our toddler he does a pretty good job of his teeth on his own but I might help to make sure he gets his back teeth cleaned properly. Good teeth care should start young.
You're supposed to start brushing their teeth as soon as they errupt. We started with a clean washer and a finger rather than a toothbrush. With my daughter we used to try and forcibly brush her teeth when she was little and it was horrible, so we moved to trying things like brushing each other's teeth and gradually she has learned to brush her own. With my son we have let him learn to brush his own by watching us and his big sister. His technique's not quite there yet but he's getting in to good habits and actually likes doing it which is easier on everyone. The kids don't use toothpaste yet, just a brush and water and a rinse afterwards.