As the last poster suggested, ramping up slowly is important to allow your body to (literally) get back into your desired shape. If you are staying home with baby, then you may find it most practical to do brief, intense, body weight crossfit-esque routines during naptime (then, when you do get some me-time you are actually free to do as you like). Lots of inspiration on crossfit websites out there, here's an example: 3 rounds for time of: 20 push-ups, 30 sit-ups, 40 air squats. You're done in ~10 minutes and the sweat is flowing freely. That burst of exercise will stoke the fire for hours.
If you don't want to do your own programming, then buying a kettlebell and instructional DVD might be perfect for ya. I've heard that DragonDoor is the place to go for that. A simple kettlebell can produce amazing results.
Remember: what you eat can boost or destroy all the exercise work you do. Nobody can outrun or outlift a poor diet. I suggest going strict paleo for a couple months. My wife did this after our last child in 2010, mixed in a little training, and now she's hotter than when I met her 17 years ago.