W.H.
When your kid is ready! The way I was told how to know when my kid was ready was when he could wake up in the morning still dry - that's when they have bladder control and can hold it until they get up and go pee.
The other signs of readiness is an interest in the potty process - watching other family members/peers, sitting on it, etc.
Certainly introduce a little potty seat (although ours never got much use!!) and sit your kid on it just before baths, right when they wake up, etc.
And - know this - you can start potty training a kid at 2 and the potty training process can take a whole year or two, or you can wait until the kid is 3 1/2 and the potty training process can take only a few days. It is ALL UP TO WHETHER THE KID IS READY.
When my son was born, I had friends who had lots of boys and they all said boys pottytrain about 3yo, not to bother trying before then. Girls can be pottytrained earlier, like around 2 or so -if they're interested. When I moved to another state and made new friends who also had sons right around my son's age (he was 2 3/4 then) I was surprised and yes, I admit to feeling a little embarrassed or feeling some 'peer pressure' because 3 of the boys were already pottytrained (one did it on his own, right when mom was having the 4th baby, she was surprised, the other two I dont know/remember how/why they pottytrained then)
I stopped feeling bad that my son was *still* not pottytrained at 3 1/2 when one day I was out with one of the boys at a playplace and he had an accident. (He was potty trained for daytime peeing only, not poop or at nighttime either.) They didn't have anything, no extra clothes in the car, so I offered a diaper from my car because I didnt want the poor kid to go half naked in a playplace!! I dont think extra-early potty training does any good.
I would MUCH MUCH MUCH rather deal with diapers than poopy underpants!!!
Another tip when you're actually in the process - no pull-ups (maybe nighttimes, or if the kid really wants to use the toilet) Get those thick training underpants so they are actually WET, not just wet-diaper wet. (or some others swear by the 3 weeks - $300 method - which is 3 weeks of no pants/diapers/etc and then $300 for carpet cleaning after the pottytraining process is over!)