If you have previously been pregnant, even for a few days, your body has had the opportunity to form more of the milk-making cells than you had prior to pregnancy. These milk making cells will allow your body to produce larger quantities of breast milk than if you had never been pregnant. If you haven't been pregnant, it will still be possible to produce milk, but the quantity may not be enough to provide only breast milk beyond the first days or weeks. Supplementing your milk with infant formula may be necessary. Breastfeed the baby first and finish the feeding with formula.
Prolactin is the hormone responsible for the production of breast milk.
Prolactin is produced in the hypothalamus of your brain and appears in your bloodstream. The milk making cells of your breast are reactive to this hormone and respond by producing breast milk. In the non-pregnant state, your prolactin levels are not elevated. Normally, when a baby is born, the separation of the placenta frees the system of an inhibitor which blocks prolactin. Without this inhibitor, the prolactin levels rise and the production of breast milk begins.