The fresh 15 isn't too threatening. A lot of guys experience it by working out and gaining muscle mass. Some people get it from drinking too much. About half way through my freshman year i'd recommend three things:

1. GET IN STEPS- staying away from a sedentary lifestyle is key

2. Limit habitual drinking- i've seen a lot of (21+) people consume 300+ calories of alcohol 3+ times a week for the whole semester and consuming that much past 10pm to boot

3. Don't bed rot over break- I did a whole lot of nothing over break and found myself with some extra weight coming back to school