Most Helpful Girl
looks like you are a naturally skinny guy. you need some roids :P1
Most Helpful Guy
You have some decent definition.
A couple of things if you're wanting to get bigger. Firstly, get in the gym and lift heavy, not just bicep curls and tricep push downs but build your routine around compound exercises for the major muscle groups. Secondly, make sure your diet is right and you're getting a level of protein appropriate for the amount of lifting you're doing.
Do those things and, especially at your age, you will start packing on more mass. The advantage of a naturally slender frame is you can do that while maintaining good definition too. I speak from experience.1