Trying to lose weight...

I am trying to lose weight and I'm breast feeding. Is it okay for me to change my diet? Also can I take any supplements to help lose weight while breastfeeding? I just got the okay from my doctor to start working out since I had my son. I'm already at my prepregnancy weight but I look fatter than I did before I was pregnant. :( if anyone has any advice on how to lose the stomach fat It'd be greatly appreciated