Exercise... Cardio (running, swimming, biking, etc) can be done every day- I recommend taking 2 days off per week. Start lifting weights- if you don't have access to weights go back to the basics with squats, push ups, crunches, bicep curls w/ soup cans. Weights should be 3-4 times per week and make sure you rest a day in between sessions!
Eat lots of veggies, fuel your workouts with a complex carb like steel cut oatmeal or whole grain turkey sandwich or an omelette an hour or more before the workout. Eat small mealls through the day- 6 mini meals. Keep the metabolism burning.
Hope this nutshell version helps!