How can I not gain weight in college?

I noticed that I might have gained just a little bit of weight in college. I don't drink, and I usually only three meals a day in the cafeteria, and I don't eat alot even though it is an all you can eat. I snack sometimes, because after dinner I always crave something sweet. I don't know why. I don't snack or eat chips or anything during the day though. I do eat pop tarts for breakfast though, but that is kind of my only option considering I have 8am cles. Considering all of this, what can I do better? Cut out the late night snacking, and if how are there any other healthier alternatives to satisfy my sweet tooth?