When do you start to train your child to use potty?

You can gauge if your child is ready for it. If he is able to sit up right and verbalize pee or poo (depends on how you say it at home). He could also be pointing out at his diaper when there is something at it. In short, he is able to express himself, not solely verbally. You can buy a baby potty and show him with a doll and have some runs at it, even with the diaper on. Then gradually make a routine for it. Say, in the morning after waking up. There are also potty training books for boys and girls with photos of children in the potties. This might encourage him. And of course, it takes time and patience. Good luck! :-)