When is it ok to start work-out after delivery? any special precautions to take?

You cannot diet if you're feeding. The baby is getting all it's nutrients from you and needs every bit of them. If you diet, the baby diets and babies shouldn't diet. Just remember, What you eat (or don't eat) The baby eats (or doesn't). As for exercise, I've always waited till after my follow-up appointment at 4 weeks after. Talk to the pediatrician about the diet thing. I'm sure he/she will tell you the same.