i'm 17, 5'7 and 160 pounds. I would like to lose 20 pounds, to be at 140. i recently started walking for 30 minutes a day, and stopped drinking sodas. I now only have about 1-2 diet ones a day, compared to the 6 nondiet ones i was having. What should i eat, and what exercises should i do to reach my goal. i REALLY need to work on the stomach but everything i do, never seems to work right.