Nurse and nurse often, pump after you nurse, and always pump if you miss a nursing session. Drink plenty of water. You might also want to try eating oatmeal every morning or as a snack (instant oatmeal, oatmeal snack bars or cookies, etc.), it helps produce milk. I noticed a difference in my milk supply within a few days of doing extra pumping sessions and eating oatmeal.
If that doesn't work... some people will recommend fenugreek. I'm very leery of herbs, you should do your research before trying it and talk to your doctor. You can't take it if you have certain health issues, risks, or take certain medications. Herbs aren't regulated by the FDA, so you never know what you're going to get. Here's a link on fenugreek: http://www.kellymom.com/herbal/milksupply/fenugreek.html
You can also get a prescription from your doctor to help with your milk supply.
You might also want to check out the Le Leche League: http://www.llli.org/WebUS.html. Contacting a lactation consultant would also be a good idea. If you don't already have one, your doctor can probably recommend one, or the hospital where you delivered probably has one on site. http://www.kellymom.com is also a fantastic breastfeeding resource website.
Hope this helps. Good luck!!