Losing weight while breastfeeding?

Does anyone have tips for losing weight while breastfeeding and keeping up your supply?
I've been doing some research and a lot of people are saying that the 21 day fix is really good.. But, as a single mother of 2, it's going to be hard for me to afford to follow that diet plan. 
I want to eat healthy, but as cheap as possible. 
My plan was to count calories with my fitness pal, but I don't want to not have enough calories and effect my supply. 
HELP??