You'll need to wait until your 6 week post natal check to start exercising, but until then the best thing you can do for your body (especially if BFing) is to eat healthy. Make sure you're getting a good amount of protein and good fats to help milk production. You shouldn't avoid carbs, but just make sure you're being healthy with it (ie a quinoa salad or sweet potato chis rather than a chippy). Carbs are important while you are BFing and remember that you will need an extra 500 calories a day.
Once your post natal check is ok, then the best thing I can recommend is the gym while your OH watches LO. If a gym isn't for you, then look in your area for abuggy class where you can take LO with you. If there is a pool in your area, check if they do an post natal class.