I've been following a diet my mate gave me about a year ago.
He went from 18 stone to 13 stone in 6 Months!
I've been a bit more slack with it but have managed a drop from 19st 10lb, to 16 stone 10lb.
I do a fair bit of weight training as part of the program so some weight I've lost in fat has been gained in muscle.
Its basically a low carb, low fat, high protein, high fibre diet. Along with around 60 - 90 mins in the gym 3 - 4 days a week.
Everyone will give u different types of diets but in its simplist form your intake of calories needs to be lower than the amount you burn. Its just finding a method that suits your life style.