You are still going through puberty. And will do so until your late 20s (approx. 28, or so, when the pituitary glands begin to slow down the growth of your bones, muscles, tendons, nerves, cartiledge, etc.)... a truth that almost all people don't know. Just because the government, and society say you are a man at 18, able to vote, and die for your country, the body doen't finish growing until almost the age of 30.
So, you are in a perfect position to gain size. Not only are you growing naturally, but by starting working out now, you can grow twice as fast as a normal young adult, or someone older, starting working out later in life (like me, LOL...)
Concentrate on working out your entire body, as already suggested. Educate yourself on the body, what to feed it, what NOT to feed it, what to supplement with, and the amount of recover time needed. Search out a reputable, certified, trainer if you need assistance learning the routines. DO NOT DO STEROIDS at your age. That will signal the pituitary glands to slow down early, or stop producing entirely, the hormones that make you grow naturally.
With your body style now, ANY work outs you start, will yield results VERY quickly. You have no fat reserves to loose first, to show the definition as you grow, as older, heavier, people must loose. So you will begin to show size and definition very shortly after starting....
Good luck, keep all your RJ brothers informed!