I don't believe, personally, that green tea helps with weight loss. I am an avid herbal tea drinker-and whether I am drinking green tea, black tea, traditional iced/hot tea (pekoe), or other specialty blends-I have never noticed a difference with "green" tea as far as weight loss. Teas do have nutrients and antioxidants and things in them that are wonderful for the body though, in other ways. (I do avoid all "diet" teas which contain ginseng/Ma Huang or other herbal "speeds" in them, or ingredients like senna-which is an herbal laxative)
I don't really believe that the Asian population is thin mainly because of tea drinking, either. They also walk or bike more/drive less, eat more soy, veggies, and whole grains than we do and less processed foods-and have other healthful habits. Genetically, they are also smaller boned/framed than a lot of us from other descents. In recent years, though, there has been a rise in weight problems in Japan, etc. due to more fast food chains and things popping up over there.
As far as metabolism-I recommend EXERCISE!!!!!!
For best results, what you want to do is have a regime that combines:
~Strength Building Exercise~ Use weights or do Pilates or some other resistance exercise to build lean muscle mass. Lean muscle mass will give you better abs/legs/arms/butt/etc. and that muscle will burn more calories throughout the day while you are doing your regular every day activities just EXISTING.
~Cardio Exercise~ Cardio (such as walking, aerobics, dancing, or what have you) burns a lot of calories WHILE you are doing it, and helps to burn fat.
The result of doing both forms of exercise, is that you will burn a lot of extra calories-a lot DURING your cardio workouts, and a lot around the clock from doing resistance/strength exercise. The resistance exercise will firm and shape your muscles (You can get smaller with diet alone, but only strength exercise will give you flat, tight abs or a more compact butt!) and the cardio will help to get rid of the fat layer that is "covering them up".