My alt Spriest had dinged 71 and despite hardly ever seeing a dungeon on the way to that I'm all of a sudden really keen to take it to Northrend dungeons! The problem is, I haven't a clue what I should be doing both in terms of DPS and group interaction.
I'm more than fine in solo PvE (infact it seems pretty easy to take on multiple mobs of higher level) but don't want to make a complete a**e of myself when I walk into UK or Nexus for the first time!
Shadow priests don't really have "rotations" anymore. It's more of a priority list:
1. If your Mind Blast is available (not on cooldown) cast it.
2. If Vampiric Touch is 1.5 seconds from fading, re-cast it.
3. If Devouring Plague has faded, re-cast it.
4. If none of the above, cast Mind Flay.
Your opening rotation should be something along the lines of:
Vampiric Touch, Devouring Plague, Mind Blast, Mind Flay, (5 stacks of Shadow Weaving is now on the mob) Shadow Word: Pain.
Mind Flay refreshes your SW:P duration on the Mob with the Pain and Suffering talent. That's why re-casting SW:P isn't in your "rotation."
For trash packs of 4 or more, just spam Mind Sear. 3 or less, do your regular rotation on each mob until they die, then move to the next.
Once you feel you've got the hang of it, a really useful mod to download and use is called EventHorizon. EH allows you to really line up your casts and dots perfectly so that you never screw up that priority list or waste a single global cooldown. Another advanced skill it helps you master is clipping your Mind Flays after certain "ticks" so that you can perform another action higher above in the list (Mind Blast, refresh a dot, etc) in the middle of a cast. But again, this is all min/maxxing for raids, so don't really worry about that until 80.