get a heart rate monitor and do some sort of walking / treadmill / bicycle etc that pushes your heart rate into "fat-burning" range. For many adults this is around 110-120 beats per minute. Start with what you are able to do and build up to at least 30 minutes a day, six days a week.
A good book is "Younger Next Year", which describes how and why all of this can help and why it's important. It helped me lose 35 lbs in six months.