Bring 3 Marks of the Valiant to Clara Tumblebrew at the Argent Tournament Grounds.
If you lose your lance, you can get another from any of the Lance Racks near the stables or in the Silver Covenant Pavilion.
The grand melee is the most important of the tournament's competitions. The people of Ironforge are looking forward to hearing of our success. To that end, they've been honing their skills against the valiants representing the other Alliance capitals.
I suggest you do the same. The valiants practice in the ring just outside our tent. Equip your lance, go to the stables outside the pavilion and mount the stabled Ironforge ram, then challenge other Alliance valiants.