Start weight training. Your male androgenic hormones are starting to peak now and building muscle mass will come readily with hard workouts for a couple years.
There's no easier way unless you view the opposite of skinny as fat and I'm sure you don't!
While you are exercising and building muscle mass, try eating healthier too. Those in training, know that nutrition is important. You probably could find a lot of articles about this on the internet.
How can i get bigger??? 
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