You'll need to learn how to break through and opponent's defenses to suceed in mounted combat. Take this lance and mount the stabled Quel'dorei Steed just outside the pavilion.
Once you're mounted, seek out Valis Windchaser. He's on the northern edge of the tournament grounds at the training area east of the Aspirant's Ring. Valis will teach you the basics of using a ranged Shield-Breaker attack to make make a Ranged Target vulnerable.
The ranged Shield-Breaker attack is simple to learn, but difficult to master. Alone it may seem weak, but in conjunction with Thrusts and Charges the Shield-Breaker makes a successful jouster.
Find a Ranged Target. Take a moment to the Shield-Breakers range...How much space you need, and how far you can hurl the lance.
After that? Well. Start throwing, and do not stop until you have broken every last layer of Defend and pierced the vulnerable target.
This tactic will serve you well in actual mounted combat: Break down your opponent's defenses...and then crush them!
After completing "The Valiant's Challenge", the following quests open up allowing players to repeat the valiant stage of daily quests (steps six and seven) for the other factions with which they have not yet earned the right to champion: