We brush our 17 month old's teeth every night after his bath as part of his bedtime routine and he won't let us forget - he really enjoys it. Since he got his first tooth at about 3 months, we've used an Oral-B stages toothbrush that the dentist recommended and the Oral-B stages fluoride-free toothpaste for babies and toddlers. We first brush his teeth as best as he'll let us (which is pretty good), and then always let him finish up - he mostly chews on the brush and sucks the toothpaste, but I think that the idea of getting him in the habit and practicing his fine-motor skills by handling the toothbrush himself is really the most important part. Then he "rinses" by drinking water from a cup, which has been a useful way to get him to learn that skill, too.
My sister-in-law whose son is the same age brushes her kids' teeth twice a day. I think that for now, once is enough for us. Like I said, establishing the habit is the most important thing for us. Our dentist said to bring him in for the first visit when we feel he can sit still in the chair. That won't be for a while, but at least I know his teeth are getting daily attention.