Many mothers notice engorgement, or over-filled breasts, at some point or the other while they are breast-feeding their baby and it is especially common to experience when your baby is first born and you are just starting to make milk. So for the first couple of days you make colostrum and then 2-5 days later your milk comes in. And sometimes it comes in with a vengeance and all of the sudden you feel really full and it can be painful and very uncomfortable.
Normally your milk supply will even out and start to work well with your babys demand, so it is kind of a supply and demand type of function, but until then, if you feel engorgement, there are a few things you can do to relieve it.
If you are nursing your baby on demand this will usually help to self-regulate and most young babies want to eat every 2-3 hours and sometimes even every hour. So, basically, the more often your breasts are emptied the more relief you will feel. But on the same hand, the more you nurse the more milk your body will probably produce. This is why it is good to go off of your babys cues because then you will make what your baby needs and hopefully not much more.
But if you are making more than your baby needs and you find that you are still full after feedings you will probably have to either manually express some milk or pump it off, so have a good pump available in case you need to, and if you don't, you can manually express the milk by gently massaging from the armpit down towards the nipple. And you can also try using heat prior to nursing your baby or pumping milk off and this will also help to relax things and help you to get the milk out. Take a warm shower and then feed your baby or use a warm compress.