For a faster metabolism all you need to do is grow some muscle! Muscle is what we call metabolically active tissue. This means that your body expends calories to maintain it while you are at rest. The more muscle you have the more calories are required.
HOWEVER, Because of this, a common thing I see with clients is that they believe once they gain some muscle they can eat a lot more food and still stay lean.
This is not really the case, research shows that the effects of muscle growth on your metabolic rate shows only a very small increase. For every 1lb (0.45kg) of muscle you gain, most people will burn an additional 6-10 calories per day.
So if you want to have one extra 200 calorie snack each day, you will need to gain upwards of 20 lbs (9kg) of muscle, which can take years to accomplish.