I have been exercising pretty regularly since my son was 3 months old, he is now 8 months. Yoga and pilates are both good for abs, although just walking helps. I work out 2 times a week (yoga and Zumba), and then do at least a couple of other things-biking, hiking, or walking during the week. My body was back to normal within 6 months (and I gained 40 pounds during my pregnancy), but I also swam or walked at least every other day while pregnant. Just know that when you are breastfeeding and exercising, you will have to eat more food. I almost exclusively breastfeed, he only eats about a tablespoon of food a day, everything else he gets is breast milk, and I am hungry all of the time and eating constantly.
Just some ideas, you can start walking a few times a week, start out small, like half an hour, and work your way up to a couple of miles each time. Try to find a friend that also wants to get in shape, definitely helps when you have the support. You can use a yoga dvd, but going to a class with others will make it more fun, and you'll probably stick with it longer. Check out your city rec center, some have great deals (we have yoga, etc. for $2 a class), and since it's getting hot, take your kids swimming. Fun for them, and you'll be moving around enough keeping up with them that it's good exercise. Also, give yourself a goal, maybe see if there is a 5k in a few months you could walk in. Good luck!