Confused about breastfeeding

If we eat more we produce more milk ...is it true or false or myth.my baby is 1 week old am not getting enough milk ..so I am using formula n my milk as well .my problem is I want to burn some colories with breast feeding.am doing little exercise also but my grany told excercise will decrease my milk supply.and I should eat more to produce milk.is it true.....I don't want to put weight like my previous pregnancy..suggest me if u tried anything and worked for u to improve milk supply