Studies have shown that improving one's diet is a very effective way of losing weight, especially when paired with regular exercise (30 minutes per day is recommended, with one day of rest per week).

You don't have to completely overhaul what you eat. Just reduce your processed sugars (most important, as processed sugar is very dangerous to the body), simple carbs, empty calories (fast food namely), and fats. Go for complex carbs, fibrous meals, stuff that will fill you up and work with your bod to make it healthier overall. Drink lots of water as well.

Also it's a good idea not to eat at least one hour before bed, or in the middle of the night, since your digestive system isn't working to digest your food. Usually when you wake up at midnight hungry your body is actually thirsty, so keep a bottle of water by your bed.

Finally, make sure to get plenty of sleep- at least 8 hours per night. 10 is optimal if you're a teen.

Once you start eating just a little bit better, sleeping a bit better, and doing a bit of exercise, you'll find you have more energy to actually go out and exercise more. Your body is actually working with you on losing weight. You'll be stronger and healthier overall, and more likely to keep weight off than if you just exercised forty hours a week.