It is really cool having those broad shoulders; makes one look bigger and better in all kinds of wear. This is how you can achieve it.
Focus on the medial (or side) deltoid muscles but also include front and rear deltoid work. The side deltoids are the furthest out on the body and hence when developed, really create the broad shoulder look. Side lifts with dumbbells, arms slightly bent, with a slow release, will target the medial deltoids.