All the tool tips in game say what to use centering for and if it builds/uses centering.
The tool tip for Zen says that it converts 30 centering and lasts for six charges, and that each form has a different effect:
Shii-Cho - Slash costs no focus and strikes one additional nearby enemy.
Juyo - Increases the critical chance of your burn attacks by 100% and causes burn damage to heal your party for 1% of max health each tick
Ataru - Reduces focus cost of slash and blade rush by 1 and reduce their global cooldown by .5 seconds
There are also talent points you can use to improve how quickly you gain centering points. In combat there is "defensive forms" which with 2 points in it says it will build 2 centering when attacked and that the effect only procs every 1.5 seconds. There is also Temperance which will give you one centering for every enemy you kill.
In Watchman there is Valor which gives you one centering when using abilities that spend focus.
In Focus there is Inner Focus which with 2 points into it gives you ten centering when Transcendence ends.
It's all pretty much explained in-game on the tool tips, you guys just have to read them.
I do want to point out in this thread that the tool tip for Inner Focus is incorrect (or it's bugged). It says that with 1 point in it you will gain 5 centering when Transcendence ends, and with 2 points in it you will gain 10 centering when Transcendence ends. This is incorrect. With 1 point into it you will get 10 centering, and 2 points into it you will get 20 centering. I'm Focus specced on my 35 Sentinel and I basically spam Transcendence nonstop since I build centering so quickly.
Hope this helps some of you.