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 Silver Covenant 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 to you.
It would be wise to continue practicing your skills against the combatants waiting in the Ring of Champions. Equip your lance, go to the stables on the southern side of the grounds. then mount up on any mount you're permitted to use, and face the champions.