I agree with LJ.
I'm nursing my 10 month old and started counting my calories since he was about 3 months old. At that time my weight was somewhere in the 180's. Today when I stepped on the scale it read 133.4.
I'd suggest keeping a food journal and trying to eat as many fruits, veggies, lean proteins and whole foods as you can. Because you're nursing you can get away with a relatively high number of calories compared to other dieters.
Just to give you a ballpark, I started by lowering my calories to 1850, then slowly decreased that to 1700. I had gone as low as 1600-1650, but that wasn't enough for my body and I ended up binging a lot. You might have to play with how much you eat at first, but don't be afraid to eat a larger amount of calories, your body and your baby need it! And of course, lots of water/fluids.
Also as a caution, I did experience a marked drop in breast milk production when I started eating a calorie deficit. I still had enough for my baby, but I didn't produce much extra.