I'm fifteen trying to gain some weight through muscle but look good?

I'm fifteen, 6 foot one, 140 pounds, I'm really skinny and tall obviously. I have 35 more days of summer before new school year starts. I'm pretty handsome, I'm not a knockout but yeah. In some pictures of me I kind of look like I'm slouched a bit, I think it's because I don't have enough muscle in my back and chest. Talk to me, workout plans or something...