Gaining weight with a fast metabolism?

I'm 16 years old, 5'7" and I weigh ~106. I have a fast metabolism (like everyone else in my family) and even though I eat 3 square meals and several snacks a day, I don't gain weight. ( I exercise about twice a week riding horses. And I walk my dogs weekly) I'd like some tips, such as which kinds of foods are best. Besides drinking soda occasionally, I eat pretty healthy. Should I eat more junk food? I'd just like to gain a few more pounds as bikini season is coming up and I don't want to look like a skeleton. Thanks <3 No mean comments please.