Metabolism is a fairly complicated subject. I will try to address the questions I receive most often. Metabolism is affected by your body composition. By body composition, I mean the amount of muscle you have versus the amount of fat.
Muscle uses more calories to maintain itself than fat.
For example, let's say you have two people who are the exact same height and weight. One exercises on a regular basis with weights, in addition to aerobic exercise, and has a low percentage of body fat. The other never exercises and has a higher percentage of body fat. The first person who exercises will have a higher metabolism than the second person. Begin to exercise and stop dieting.