Hi Ali,
This is just my own experience and opinion, but the sooner you can start to introduce exercise into your life the better and your body will thank you for it.
Walking is a simple yet very effective way of burning the cals. Strap on an iPod, get some comfortable shoes/trainers on and away you go! DVDs at home are also great because you're in the privacy of your own home so no-one there to see you puffing and panting
Good news - lost 3lb at last nite's weighin. Woohoo.