Exercise is shown to help people quit smoking. I actually remember reading a study that stated that, of the four worst health habits for a person - smoking, poor diet, inactivity, and binge drinking - the one factor to change which results in a higher likelihood of the other habits being resolved is your physical activity level.
Find an activity you like. A sport, maybe cycling, join a gym. Really, it doesn't matter what the activity is. As long as you get a good workout out of it and enjoy it, you're likely to stick with it.