WHM is definitely the most beginner friendly of all healers. Strong heals, straightforward abilities, plentiful of oGCD heals, and great MP management. You may stumble across people who say WHM is boring to play. This is because it have plentiful of skills to get its job done so you can press that 1 dps button over and over again.

I can't say much about SCH since I don't have it leveled yet, but they provide shields on top of healing. It feels great to play when you know the encounter of certain fights so you could pop shields before damage goes off.

AST is a little bit of both where they can choose stances between providing HoTs or Shields. It is the busiest job among the 3 because on top of healing, they have to juggle with cards to buff the party. You have to learn the different card effects when starting out, albeit they are much simpler now compared to previous expansion. Then also have to align seals correctly for the ultimate (6%) party buff.