how do I get my milk supply up?

So I have to decided to just pump because it's a lot easier an works better for my fiancé an I. But when I started I was getting 2oz every pump, then it went up to 3oz that then finally to 4oz. But that was me pumping 2x a day because that's all I could ever get. But now my problem is that I'm barely getting 2oz. I really want my baby boy to have breast milk rather than the formula the doctor put him on to supplement, but I can't seem to pump more than 4oz in one day.. I just don't know what to do..