A persons metabolic rate is a measurement of how fast their body burns calories which are, in turn, a measurement of energy.
Burn more calories than your body consumes and you lose weight; burn more calories than your body consumes and you gain weight. Another way to say that is this: consume less calories than your body burns and you lose weight; consume more calories than your body burns and you gain weight.
The way to use your bodys metabolism to lose weight, then, is twofold:
Consume less calories than you burn (as in dieting);
Burn more calories than you consume (as in exercising);
The average bodys metabolic rate, and the rate at which the Percentage Daily Values of the nutrients on a foods nutrition label are based, is 2000 calories per day.
Your body burns a certain number of calories per day just by virtue of you being alive I other words, no exercise necessary. The process of breathing, for example, burns calories. As does the circulating of blood. All forms of burning calories based on nothing other than the act of being alive make up whats called you Basal Metabolic Rate (BMR). Interestingly enough, a persons BMR makes up about 60% of the calories their bodies burn in a given day. The remaining 10% of the calories burned in a day are burned by whats called Dietary Thermongenesis, comprising the heat-producing acts of eating and internally processing food.
It is entirely possible to boost your bodys natural metabolism, in other words: burn more calories in a shorter period of time, or burn calories faster. Strength training (as in lifting weights) is one way to achieve remarkable effects. When you lift weights, the muscles tear and must be repaired by the bodys natural healing processes. Those processes require the body burn more calories. The body with a bigger muscle mass burns more at rest calories in any given span of time. As a general rule, we burn 50 extra calories a day for each extra pound of muscle.
Beyond weight training, the other tried-and-true method for boosting your metabolism is simply remaining active. The mere act of moving around burns calories, so logically the more you move around the more calories you burn. Take walks on your breaks at work. Park one or two streets over. Tidy up around the house while you chat on the phone. None of these require a gym membership or an unscheduleable block of time. A little bit over a long time adds up to a lot.
Lastly, there are dietary measures you can take to help boost your metabolism. Caffeinated beverages (green tea being one of the healthier choices) and spicy foods have both been shown to increase the bodys metabolic rate. Also, if you start having more frequent smaller meals rather than jus 2 or 3 large ones (grazing throughout the day, so to speak), your metabolism will be in a constant state of activity breaking down and processing your many small meals.