Exercise is great, but if you're gaining weight, you'll need to cut calories. Limit your alcohol, soda, and fast-food consumption. Substitute healthy foods like fruits and nuts for sugary snacks. Incorporate more fiber (whole-wheat bread, fruits, veggies) into your diet. If you're eating in a campus dining hall, do not take advantage of the all-you-can eat policy. I have no data that supports this, but I suspect the low-carb diets work primarily because they remind people that sugars are carbohydrates.