Bring 4 Marks of the Champion to Luuri 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 Sunreaver Pavilion.
As a champion, you have mastered the art of mounted combat, besting challengers of all levels. When the tournament begins, those you have defeated will have redoubled their efforts and will seek to avenge their losses.
It would be wise to continue honing your own skills and learning from your opponents. Champions representing all cities practice in the Ring of Champions. Equip your lance, mount up on any mount you're entitled to use at the stables, and face the champions.