As Linear2 said, walking is a good start. Then, when you feel fitter, finding out if you'd like other activities. Also as Linear2 said, anything you decide to do should be gradual.
Diet is important but balance is a key there, fruit and vegetables are good to use as snacks (and meals). Having celery and carrot sticks with low fat cream cheese can be a healthy snack (in moderation).
Grapes, strawberrys and other small berries can be carried easily in lunchbox/coolbox and are a good snack (but are not so filling unless eaten with raw nuts, like walnuts or peanuts). Avoid salted, sugared and cooked nuts as they are higher in fat but otherwise nuts are good for you (unless of course you have a nut allergy).
Hope this info helps.