The short answer is to eat less junk, and do more exercise.

Cut out the junk, sugar and processed foods, eat more protein, healthy fat and green vegetables. Eat only when you are hungry.

Be more active. Walk as much as possible, do something that makes you sweat occasionally, lift weights or do squats and press-ups.

Turn off the TV and record your favourite programs to watch later. Limit internet time. If you sit at work, get up and walk around frequently. Go to bed early and get plenty of sleep.