Other things you can/should try: get 8 hours of sleep, drink more water and less soda, stay away from bizarre diets, and you must become more active! You cannot achieve a higher level of fitness without getting off the couch. Walk an hour, bike an hour, play basketball for an hour, train in your martial arts class for an hour, lift weights for an hour . . . get the picture?
15-20 minutes of exercise might be a popular idea or movement, however, it won't get you the results you want. An hour of intense exertion several times (3-5) per week is what you want.
Now before you get discouraged, just try one of these things. Perhaps you'll try cutting out soda for a month. Perhaps you are going to try and do some sit ups and push ups. Just do something!
We're all capable of achieving great things, we just have to decide to do them.
If you are not in a uniform yet, come see me and we'll get you fixed up for classes, you'll be glad you did, guaranteed.