Weightloss and exercise post baby

[MrsFabuloso]

It has only been 3 weeks since I had my little man, so I am not stressing too much over weightloss just yet, but in saying that I am at my heaviest I have ever been, being about 89kg and only 165cm tall. That's pretty heavy. And I'm really unhappy with what I seein the mirror. And uncomfortable.
Now I know its not important at the moment, we are still settling in with DS, but I would like to know what I can do now to begin being healthier. I am trying to make an effort to eat well, not just for me , but for DS since we are fully bfing at the moment. What else can I do? I had a pretty 'easy' birth (if you can call birth easy!) and have stitches for a tear, but that's it. Can I do exercise yet? If so what could I do keeping in mind I have two babies under 16mths.

ETA I should probably add that my core muscles and even just major muscles (thighs,calves,arms...) are REALLY weak, so anything that will strengthen them first would probably be best! And I have 3 flights of stairs to go up and down with 2 babies.....