The first healer I made was a druid. Pretty easy to learn, a lot of instants. Just keep your HoTs (healing over time) on the tank and it should be pretty easy to pick up. Resto shamans are really simple to learn as well.
If you really want easy mode, you could make a discipline priest. At lower levels all you need to do is shield the tank and sit and wait until they take damage, lol. When you reach high enough level to unlock atonement, you can use smite/holy fire or solace/penance on a mob and it will deal damage but the damage will convert to a heal to whoever is lowest on health.
All healers work well though. I just found druid/shaman to be especially fun while leveling up and in 5 mans. And I have one of every healer at 90, including two different priests.