From about the sixteenth week of your pregnancy you breasts will have started to secrete early breast milk, colostrums. After giving birth your baby and the placenta tell your body to start milk production. Your breasts will become larger and fuller as soon as your baby is born - this is a natural process and will happen irrespective of whether you plan to breastfeed or not.
A hormone called prolactin is what makes breast milk, and if the milk is not used, either by feeding your baby or expressing it, then your body will secrete prolactin inhibiting factor, or PIF. PIF lets your body know that the milk is not needed, and this in turn makes your body begin to stop producing milk. This process takes about one week after your baby is born if you are not feeding your baby or expressing your milk. Once this happens, your body will stop the lactating process, and you will return to a state of non-pregnancy. You might feel uncomfortable during this time because your breasts might become engorged with milk. Should this happen, wear a supportive bra for comfort, but do not bind your breasts - this could add to your discomfort and there is a risk of mastitis and plugged ducts.
Taking a non-steroidal medication such as ibuprofen will help as will applying cold compresses or ice packs to your breasts. Although doing this will not stop milk production, it will make you more comfortable. If the discomfort becomes too much for you, you can express a small amount of milk to ease your breasts, bearing in mind that expressing will tell your body to produce more milk, which will lengthen the time that it takes your body to stop producing milk. Another thing to remember as well is to not let warm water run on your breasts while you are showering, as this will stimulate milk production.