The only way a diet can work is if you are burning more calories than you are eating. You can use the Calorie Calculator to figure out how many calories you burn each day. Once you get an estimate, all you have to do is eat less calories than the number the calculator gave you.
If you are tracking your weight and realize you are not losing any, there are two things you can do. Since there are two parts to the losing weight equation: calories burned and calories eaten, you can change each part of the equation to lose more weight.
The problem, if you are not losing any weight, is either you exercise too little or eat too much (or a combination of the two). The only way to fix it is to exercise more and eat less.
If you think you are exercising enough and eating the right amount, try counting your calories to see if what you think you are eating is correct. We tend to under estimate the amount of food we take in so try counting your calories for two weeks and see if it helps.