Champion of Sen'jin

These champion achievements aren't entirely obvious.

(1) In the main building, you need to complete the "The Valiant's Charge" for the particular faction. This will require 25 Valiant Seals. You can get these by doing the dailies @ The Argent Tournament Grounds.
The Valiant's Charge
(2) You will then get "The Valiant's Challenge".
The Valiant's Challenge
(3) Find Squire Danny (68,20):
[npc=33518]
*NOTE* - You must be on a jousting mount with the lance in slot. Otherwise Squire Danny will not give you the chance to fight.
(4) Argent Champion shows up and talks to Squire Danny.
(5) Battle Begins.
(6) Immediately "Charge" (3) champion. Activate "Defend" (4). Then keep at a distance and click "Shield Breaker" (2). I do the latter by just running away at an angle. Any time the Champion is at a distance, activate "Charge" (3) again. It's a very easy fight if you do this.
(7) Return to main building to cash in quest to get the achievement.

Note - This achievement takes a while because of the 25 Valiant Seals needed.

I started one day worth of these dailies, and now I want to stop and switch to Orgimmar. how is that done since none of the other city factions will talk to me?

You can't stop. Once you have started the process for one faction you have to stick to it until you are a champion with them and then you can start with another city.