^^^Good for you bud! I love seeing younger guys and girls get into and continue to exercise. Whether its bodybuilding, running, biking, whatever; just being active.
If there was one old adage I could tell you about putting on size, and ask you to always remember, it would be that you don't get big in the gym; you get big in the kitchen.
Gaining and losing weight is simply about calories in vs calories out. You have to eat in a surplus which can be hard to do sometime; I know. Find out what your maintenance is, and eat at least 500 cals over that every day. Peanut butter, oats, chicken (you may be sick of chicken...lol), rice, yogurt, cottage cheese...eat everything!!
A great whole food weight gain shake I enjoy when I'm on bulk is:
1 scoop of whey
oats
nuts...any kind; almonds, walnuts, peanuts wutevr
banana
3-5 tbsp peanut butter; I use a natural pb like Teddy
top off with milk in a large cup and mix with a wand mixer/hand blender
This has good calories, carbs and protein and is easy to make. And you can leave out the whey if you want; it has plenty of protein without it.
Keep at it Meatloaf!