J.
Milk is produced by supply and demand. The baby's nursing at your breast is actually what stimulates your breast to produce more. That's why it's so important to ALWAYS nurse when your baby wants to and NEVER take the baby off the breast before they're finished eating. This is called demand nursing and will ensure that you will always have enough milk and not have to supplement at all.
I suggest you also look at www.kellymom.com for great breastfeeding advice as well as getting together with your local La Leche League who can help you if you get stuck! :)
Good luck!