Equip your lance and mount the Stabled Sunreaver Hawkstrider. Speak to Valis Windchaser and use Shield-Breaker on 2 vulnerable Ranged Targets, then return to Galathia Brightdawn at the Argent Tournament Grounds.
You'll need to learn how to break through an opponent's defenses to succeed in mounted combat. Take this lance and mount the stabled Sunreaver hawkstrider just outside the pavilion.
Once you're mounted, seek out Valis Windchaser. He'll be on the northern edge of the tournament grounds at the training area east of the Aspirants' Ring. Valis will teach you the basics of using a ranged shield breaker attack to make a ranged target vulnerable.