It depends on the persons metabolism (whether its fast or slow). Eating right is a main factor. Remember this: Protein Diet. Cut out all refined (processed) carbohydrates and sugar, coffee, energy drinks, etc., although they give you energy. Cut out all junk food. If you are not doing any sport or cardio exercise to work off the sugar, its just going to accumulate as body mass. You will probably feel less active for a week or so as you go through refined carbohydrate withdrawal but it will wear off. Always check with your physician before radically changing your exercise or dietary habits still give you energy. you will have to speak to a physician about these. In addition to eating healthy foods, exercise everyday 1 to 2 hours. Please don't turn to pills!
Go Gym everyday if its possible.
don't eat any snack foods e.g crisps, chocolate ect. and instead eat fruit.
instead of chips eat Sandwiches.
cut down on how much salt you put on your food.
drink water instead of fizzy juice.
try to get at least an hour of exercise every day.
if you think you're hungry, drink water and wait half an hour and see if you're still hungry, if you are have a piece of fruit.