I'd suggest Scholar if you're new to healing, if only because it covers both your healing and DPS needs. Learning to heal with as WHM requires a lot of attention to everyone's HP meters, while SCH mainly only needs to pay attention to the tank in most light parties. AST and WHM healing largely the same "watch everyone, focus on tank"

If you normally do PUG's... definitely SCH.

I'm not fangirling over SCH, I mainly play WHM, but I find that SCH is better for "DPS" players to get into rather than WHM/AST. WHM/AST's DPS-capability is entirely dependent on how non-squishy the tank is. A lot of Tanks do "all STR" which makes them super-squishy.