When your child starts showing an interest in the potty. Start introducing your child to the potty, let him/her see what it's like. Don't rush or push your child into learning and whatever you do, do not get mad! It's scary time for them.
Have you looked into the 3-day method? There's a great e-book (I can e-mail it to you). It worked really well for us! We could see it on DS's face when he was going. It's not about verbal communication. Even though he's pretty verbal now, sometimes we know before he says anything.