You need to make your deltiod muscle bigger to make your sholders bigger, this is the muscle at the outside top of your sholders. To do this you will need to do three things, excercize that muscle, eat a healthy diet with enough protien and vitamins and get the proper rest.
Excercizing to gain mass is something you'll need to experiment around with but a place to start is to warm up by getting the blood flowing to your deltiod muscle with slow easy motions using little or no weight. Then stretch the muscle gently, no bobbing. just stretch and hold while still remaining comfortable. Then slowly and methodiccally you will need to strain the deltiod muscle until you can only do the excercize five or six times, but gradually work up to that. Some different things you could try is the milatary press with barbell and dumbells. Another thing you could try is putting a dumbell in each hand and raise your hands up and down slowly as if you were flapping your wings. Then go back to useing lighter weights to warm down and then do some stretching. Do this two times a week in order to give the muscle time to recover and grow. Give yourself a really good workout once a month.
You will need to eat healthy or your muscle won't be able to rebuild itself very well, enough said about food.
Rest is just as important as eating right and excercizing because this is the time your body uses to rebuild itself stronger. This not only means getting enough sleep at night but leaving enough time inbetween workouts too. Going swimming once and a while for a change of pace sounds like a great idea.
