Start slow, that's all I say. Don't jump into deep water without first getting used to shallow water. Walking burns off a suprising amount of calories, so take a short walk once maybe twice a day. As you feel your fitness improving you can slowly build up the pace a little, and eventually break into a jog.

Equally important thing is to eat healthy. I hate the word 'diet'.. when people use the word diet, to me it implys they're only planning to eat healthily short term, which usually turns out bad.. IMO if you want to get into good shape you have to think long term, you have to want to change your lifestyle, and eat healthily for the rest of your life.