If ur talking leveling a healer, trust we got it good nowadays, I mean while leveling single target my healer spec can almost dish the same dmg as my tank spec. Which is nice.
I remember leveling my disc priest with increased dot dmg talent 5/5, increased wand dmg 5/5 and increased shield absorb 5/5. The only thing I would do was DoT - Shield Self - Wand mob to death.
As far as learning to heal before doing endgame content, I dunno if the bronze-silver-gold healer thingies are still available but those are a good start, also do normal dungeons to practice. That should learn you the basics pretty fast.