WHM is probably the easiest, and the only one that starts at level 1. If you want to play SCH you have to play ACN, a DPS,to level 30 first. Between the 2, just pick one, they don't play much differently at this point, SCH just has a lot more oGCD heals attached to it, and fairy management was gimped so that complexity is gone as well.

For AST, you can try it if you want, it's certainly the prettiest of the 3, but it's also the most difficult ,and least rewarding (according to most) to play.

Honestly, the best advice I can give is try them all out, see if you like one and play it. None of them are that difficult. I also just have an incredible negative bias towards healers atm after the changes from this expansion, so take my opinions with that in mind. The best advice I can give you as any healer, you don't need to heal as much as you think you do, learn to balance DPS and healing, the sooner you do the better off you'll be.