How can I gain weight?

I hardly ever gain weight, like I eat a lot but I dont gain much weight. Plus I lose weight when I'm stressed and lately I've been very stressed. Plus I gain it back slowly to my regular size when I'm not stressed anymore but it takes a lot. So now I look skinny and I don't like. I'm 17 and yea, I've never had any bulge in my stomach, even when I was my regular size. So how can I gain weight, but still dont get fat in my stomach.