Meat protein - burgers, bacon, sausage, roast beef, chicken, etc.
Eggs have protein.
Drink milk.
Eat an extra meal or 2 per day. Have parents make a big breakfast for you each day.
2013 son just gained over 30 pounds in 3 months doing this + weightlifting. I dont think he would have had the same results when he was a Freshman though.
He stopped eating poptarts and reduced his massive cereal intake also
If you don't have weights, you can do plenty of things with just your own body weight. Push ups, burpees, Insanity, etc.
Set a goal to add 2-4 pounds per week.
You can do it.