What is an easy way to gain weight? I'm drinking these protein drinks (Boost) but idk how healthy it is to do.

I've been thinking i could chug two with every meal, or when i'm hungry and want a snack. Would that be over-doing it? I weight like 115-120, I'm about 5'8, and I'm going into 11th grade. I'm a picky eater, and don't eat much meat besides chicken. I really could gain some weight, and I'm hoping I could find an easy way to this summer.