Basically, I cant really tell what type of body you have, so I'll give you your two options.
If you have broad shoulders, it becomes a little simpler. Gain muscle on your deltoids. Simple as that. I'm not going to go into routines or anything, but check out this website, www.bodybuilding.com
, they're LOADED with routines and information about working out the various parts of your body. Just search around and you can find the articles and stuff.
If you have narrow shoulders, it'll be a little bit tougher. You're going to have to create a pretty good illusion. Basically, you work out your shoulders , but also put emphasis on your back, to mkae you look wide. Make sure and develop your Lats (lattisimus dorsi aka your wings) with tons and tons and tons of pull ups/chin-ups and pull downs.) once you do so, you'll create a widening effect with your body. Larry Scott , one of the greatest body builders of all time and also the first Mr. Olympia, did that.