Breastmilk is produced on a supply/demand basis, so the more you nurse your baby the more milk you will make. YOu need to make sure you're well hydrated and well fed. As another poster mentioned, some people find eating oatmeal helpful too.
There will be growth spurts that your baby goes through - during those days you may feel like you are constantly nursing him. Just keep switching back and forth from one side to the other. His sucking will stimulate you to produce more and in a couple days your supply will catch up and he will stop acting hungry all the time.
If you do not do this, you *will* need to supplement with formula because you will not stimulate your supply enough to keep up with his demands.
Like many women have said here, it is not the end of the world if you wish to do this, but there are many benefits both for you as a mom and for your son if you do NOT supplement. For you these include: improved weight loss, non-stinky diapers to change, overall happiness and bonding with your baby. For him, better nutrition and bonding with mom.
There is actually no reason that you need to be pumping any breastmilk at this point unless your baby has a cleft palate or some other deformity that is preventing him from latching correctly. In the future if you want to go out for a bit or are going back to work, you'll want to pump of course. Personally, I have never fed my son a bottle - if he's with me he always gets breastfed. Our daycare provider and my husband are the ones who bottle-feed him.
As for pumped milk, here's the guidelines that I trust:
8 days in the fridge
3 months in a freezer
6 months in a deep (stand-alone) freezer
8 hours at room temp
4 hours at room temp if he's already had part of the bottle.
More info on this topic at www.kellymom.com