Speak with Marshal Jacob Alerius at the Argent Tournament Grounds to become a valiant of Stormwind.
You've proven yourself worthy of competing under the banner of Stormwind, <name>. Now, you should introduce yourself to Marshal Jacob Alerius, who leads the Stormwind delegation. As Stormwind's grand champion, he will enter you into the tournament on behalf of the city and oversee your training and testing as valiant.
It has been an honor working with you, <name>, and I wish you luck in the competition ahead.