Each encounter power you have can be used, normally, once, save for healing word.
Healing word notes:
Special: You can use this power twice per encounter, but only once per round.
Therefore, as a half-elven cleric (I'm skipping human for now), you get your racial encounter power usable once (until you can take a short rest), your cleric's level 1 encounter power usable once (until you can take a short rest), your cleric's channel divinity (once, even though there's a choice of two, because channel divinity says that you can only use one "channel divinity" per encounter), and your healing word (usable twice, because it says so.)
Honestly? I'd play an essentials cleric if the above seems too overwhelming. It was a real pain to learn when I was picking up 4e, and I don't envy you starting with one of the earliest classes written.