The best advice to anyone thinking of starting a new class is to Fail
faster. Or in non computer game dev speak, Go ahead and do it as soon
and as often as posible,

Yes your going to screw up, but the
sooner you start learning how to do it, the sooner you can start
learning to get good at doing it.the second lesson, Let anyone who gets pull happy Die. I dont care if
its the under geared Tank or the Overgeared Hunter whos going
GOGOGOGOGOGOGOGO, the sooner they realise you as a healer are 100% of
the reason they dont die to there own stupidity the better.the third lesson, Jerks happen.

Either vote to kick or leave the group.
your beter off eating the deserter debuff than to deal with a troll
prick.4th lesson, if you are not in full heirlooms and enchants, Always
manualy select the "green" dungeons, yes you will be a bit slower in
leveling, but compared to finding you you have a pull happy DPS in full
heirlooms whos bored out of his skull and has no idea how to play the
class with undergeared tank whos new to the game at even level dungeons,
going green will mean your at least decently geared and have good mana
pool for the tier.