A few things: firstly regular excersice. 45min to 1h of excersice that leaves you sweating but not incapable of speech. The idea is to burn calories and human body begins to burn fat after ~30 minutes of excersice.
Badminton, skiing, swimming, tennis etc excersices where the whole body is moving are best. Remember to keep to the limit of sweating but not breathless or incapable of speech. Slowly this limit will begin to rise, you need to increase your excersice intensity to match it. Instead of going for a fixed distance, go for a fixed speed.
Do this 4 times a week, 5 if you can. Do not eat within the hour before doing this though, a full stomach will just kill your energy.
Next: watch your food. Cut chips, ice-cream, chocolate, soft drinks etc to the absolute minimum. Best is if you don't eat them at all, but we all fall and falter sometimes so it's not too bad as long as you keep excersicing. If you are like I used to be, addicted to those stuffs, make at first two days a week a candy day: 100g max total of candies, chocolate whatever is allowed on those two weekdays. After a few weeks, remove one of these days.
BUT MOST IMPORTANTLY: Normal meals. Avoid pizza, fast-food joints, KFC and other places like those. Eat three meals a day (one of these being breakfast), plus an unlimited number of fruits and vegetables. On meals, fill 1/3 to half your plate with veggies and greens. 1/4th with fish, meat etc protein and the rest with rise, pasta, potatoes or similar. Do not go for seconds, do not eat more than what you can fill on your plate. At first, if and when you begin to feel hunger from the reduced meals, eat apples, klementines and other fruits as snacks. Try to stay away from bread that isn't full-grain. As days roll by, reduce the portions you eat at meals little by little, until you find a balanced portion size that leaves you satiated but energetic for three to four hours.
Think of it this way. Each day you have a set of points available. A full plate of rise, potatoes, meat, fish and such represents all your points. You must not go over the points limit. Every time you do, it adds to the distance you'd need to run in order to achieve the effect if you would have gained if you hadn't gone over the limit.
But at the same time, all points must be used. Not eating enough is just as dangerous as eating too much.
And in all this, veggies, fruits cost no points. You can eat as much of them as you want.
The idea behind this is to make you stop going for a candy bar or a soft drink when you feel low on energy, and isntead go for a healthy option. This can be like kicking a habit. I know from experience. But once you do kick it, everything becomes so much easier, because those candies and soft drinks contain insane amounts of energy and once you stop that influx of sugar and fats, regular excersice begins to bring noticeable results within weeks.
However, the downside to all this: there is no quick fix. Any change that is to stay must being from your lifestyle, and your body will change to mirror that lifestyle. Dehydration diets, 5-day celebrity weight-losses, magic pills that make you fit without excersice etc... they don't work. Or rather, they work for a few days or less, after which any weight lost will be regained.