there's no reason why you can't start going to the gym, just don't go crazy and overdo it.
before i became pg again i was seeing a pt once a week and found that really helpful. i also did lots of walking with the pram- uphills and everything.
after i've popped this bubba out, i'm going to join weight watchers as they have a special program for breastfeeding mothers and focus on healthy eating- no shakes or anything crazy like that.
best of luck with everything- just think lots of healthy, fresh, unprocessed food and lots of water and walking and you can't go wrong

eta- nelly is right though, your body has just gone through a lot- take your time, make healthy choices and i'm sure the weight will start to budge