When to start?
1.Does he ask for rapid diaper change to be done?
2.Does he walks uncomfortable with a soiled diaper?
3.Is his bowel movements predictable?
4.Does his posture or facial expression shows he is urinating or having a bowel movement?
 If your answer is YES top most of these, then your toddler is ready to be potty trained

How to start?
1.start when everything is settled and calm.Not when you change house/school or when you are having another baby.
2.Donot force him to learn ASAP.
3.Change diapers very frequently.

Last words but not the least
Your patience and encouragement will help this process along immeasurably.Read a story or sing a song all the time you are having him sit in the potty or do anything interesting to him.Please donot show any sort of anger.This will only complicate the process more for you.

Tips:
1.Give plenty of water before making him sit.
2.Increase fiber in his diet.
3.Wet him before he sits on the potty.
4.Do in the time when your little one is in good mood.
5.Expect accidents.(your kid will do after you have given up)