As a tank, one of your highest priorities is having the highest threat on any given mob. Threat is just another way of saying 'attention'. The more threat you have on the mob, the more attention it will pay to you.
Each action you preform on a target will produce a varying amount of threat on a target.
- Using any ability adds 1 threat
- You get 1 point of threat for each point of damage you do
- Casting buffs adds 2 threat
- Using a mana pot adds 2 threat
- The first player who initiates combat with a mob adds 1 threat.
There are 3 Knight abilities that multiply your threat.
- Holy Strike adds your damage * 5 in threat. For example, if you do a 50 damage holy strike you will gain 250 threat on the mob.
- Whirlwind Shield does damage * 10 in threat. If you do a 50 damage Whirlwind you gain 500 threat on the mob (and all mobs surrounding it since it is an AOE based ability)
- Shield of Atonement is a little more complex as it does damage + 5 * the level of talent + 1. If you have a level 5 Shield of Atonement and do 50 damage with it you gain 330 threat.

click to enlarge
So long as you keep Holy Strike, Whirlwind and Shield of Atonement at a decent talent level and use each ability on the mobs you plan on tanking, you should not have any problems with threat. Once you have used each ability at least once or twice you can use any other ability that will provide you with the best possible
damage.
The rotation you use should be based on whichever abilities are the highest level. Since threat is mostly based on damage its a good idea to start off with your most powerful (highest level) skill and work your way down. There is only one exception for this rule and that is when you are tanking multiple mobs at the same time. In that case you will always want to start with Whirlwind and use it whenever it is off cooldown.