How does one gain weight.

I'm 5'9 16m, and weigh 120 lbs, which is considered underweight. Not only this, but I have pretty fast metabolism, which makes it harder for me to effectively gain weight. Not only that, when it comes to actually sitting down to eat a meal, it feels like my appetite shrinks, and it feels hard to eat a lot.

I've been pretty skinny for a while, and don't know what I should change about my diet in order to start gaining weight. Right not now, I'm currently aiming for around 131 lbs.