Unfortunately, you can't lose weight really fast, at least not without serious health risks.
There are many fad diets around that claim to help you lose weight. Unfortunately, you are unlikely to be able to maintain your new weight, and you run serious risks of developing malnutrition and other illnesses like gallstones.
The safe weekly amount to lose is 0.5 kg - 1kg (1 -2lbs), and this can be done by healthy eating and a little exercise. Aim to eat more fruit and cut down your daily calorie intake by around 500 calories.
Don't forget you do need some fats in your diet though, good ones are nuts, seeds, tuna and mackerel.
Losing weight fast generally means you will put it on just as quickly, plus a little for good measure! This also has a detrimental effect on your heart.
Yo-yo dieting and anorexia nervosa can lead to serious heart problems. The singer Karen Carpenter sadly died of a heart problem after recovering from anorexia nervosa.
Losing weight is a great idea (if you have it to lose in the first place) but sensibly and slowly is key.