Which healer should I play as a newbie. I am totally new to healing here in wow and want to learn from the ground up.
Welcome to WoW! We certainly need more healers! 
The main warning I have to say up front is: the easiest healer is not necessarily the healer YOU will find easiest.
I am guessing you’ve played a healer before, in FF or another MMO, and if so, you may well have developed your own style, which may suit one of the other classes better.
With that warning out of the way, there is general agreement - “It Is Known” - that Holy Priest is generically easiest, followed by Restration Shaman.
The reason is that both of these specs work by letting people take damage, then healing that up. Mobs hit players. You Heal Players. Simples.
Other healing specs have different gimmicks, which often work by you setting up heals in advance of the damage - so you have to know in advance when the damage will be incoming, on which targets.
Paladin and Monk healers work by you being in the middle of the action, and require a lot of movement in addition to healing. So you can’t just stand back and assess the situation. Druid is big on Heals Over Time, so you have to place your bets early. Disc Priest needs to create shields in advance of incoming damage. And so on.
I’m not a healer myself. I tried once. I didn’t have a problem with healing,; I had a problem with parties. You’ll understand as soon as you start healing! 
So your short answer is Holy Priest or Resto Shaman. Shaman has more utility, if you want a more active style with more things to do. Priest is a more concentrated healer.
Be aware that all healers are also expected to do some damage in their downtime once you reach mid-to-high Mythic-Plus level dungeons at max level.
The best way for you to judge which you will prefer, short of actually playing them, is to go to YouTube and search for
Easiest healer The War Within
to see them all compared, and then focus in each in turn like
Holy Priest The War Within
Resto Shaman The War Within
I went with Holy
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Holy paladin is good.