Eat the right food and exercise.
Too many people have jobs which keep them sat at a desk and if this is you then you need to do 30 mins of aerobic exercise 3 or 4 times a week to keep in shape.
Start with an easy routine and build up slowly.
Cut the rubbish from your diet (all the nice stuff) eg fast food, chips etc and eat fruit, veg, fish etc.

Don't eat another pie