So you've never played a rogue in World of Warcraft before. You don't really care enough about a character that just runs heroics for badges to bother with the extremely complicated and intimidating ins and outs of min/maxing raiding rogue dps. Maybe you aren't used to playing a melee class.
Well guess what? To do reasonably good dps in heroics, (and be an asset to your group in other ways) you barely have to do anything! No extra research, no spreadsheets, no dps simulators. Just copy this spec, follow a handful of guidelines, and hit just 5 buttons for fun and profit!
Mutilate is the easiest to play. Combat requires more buttons and Subtlety sucks, so let's ignore them for now.
OK, I lied. You may need 7 buttons, depending on who is in your group, but you really only have to use 4-5 on 90% of pulls. Those buttons are, in order of importance:
Fan of Knives
Hunger for Blood
Tricks of the Trade
Slice and Dice
Garrote or Rupture (to put up a bleed so you can use Hunger for Blood. You can skip this if a dps warrior or feral druid is in your group)
Opening the fight: This is the only remotely complex thing I'm going to ask you to do. Just stay with me, it's worth it. Hit Esc to open your WoW menu. Select "Macros". Hit the button at the bottom to create a new one, and select the question mark icon from the list and name the macro "tricks". Then copy and paste the following text into the macro:
#showtooltip Tricks of the Trade
/cast [@focus] Tricks of the Trade
And drag the macro onto your action bar. Hotkey it so it's easy for you to reach.
At the beginning of every instance, target the tank and type:
and hit enter. You have now set it so that whenever you hit your tricks hotkey, you will cast Tricks of the Trade on the tank. You just became a major asset to your group because you give the tank a massive threat boost. Use this macro before EVERY PULL.
The only other concern you may have at the opening of the fight is Hunger for Blood. Keeping track of that buff and keeping it up can be annoying. The solution that's almost as effective: don't bother tracking it! Just refresh it every other pull! No complicated monitoring of the buff required.
There are two ways for you to refresh HfB every other pull. One of them is to start the fight from stealth, use Garrote, then HfB. Starting from stealth is preferable because of the Overkill talent. If you aren't starting from stealth, do a single Mutilate, then hit Rupture, then HfB. That's it. Just refresh it every other pull, and stop worrying!
AoE pulls (3+ enemies at once) are easy (and they make up most of today's heroics). Just pick a target in the pull and start autoattacking it. Then hit Fan of Knives over and over. Sure, you could squeeze out a few more dps in other ways, but it really won't matter in a heroic. Just spam FoK.
On single-target trash, you're generally just going to Mutilate until you have 4 or 5 combo points, then Envenom. You only need 4 to make Envenom worth it. If you want to ignore combo points, you can simply Mutilate x2, Envenom, repeat.
On bosses, simply add a single Slice and Dice to the beginning of your rotation. You will usually only need to use it once because the Cut to the Chase talent will keep it refreshed for you. You might need to use it again after a long break from dpsing a boss, like when the lightning guy in HoL splits into sparks or when the last boss in Drak'tharon turns you into a skeleton. Once SnD is up, just keep doing Mutilate x2, then Envenom.
Stats: every melee stat is reasonably good for you; don't sweat it too much. Put on all the agility leather you can get, gem for attack power and hit rating, and make sure you are at least over the 8% melee hit cap (though more hit after that is still good)
Weapons: Daggers. Get whatever the highest ilevel dagger is you can get, but if you have a choice get a slow mainhand (1.8 speed) and a fast offhand (1.3-1.5 speed). A nice starter offhand you can get on the AH for peanuts is Librarian's Paper Cutter.
Poisons: Instant Poison on your mainhand, Deadly Poison on your offhand. Don't forget them, and stay stocked up! You can buy them from a guy in the Dalaran Sewers.
Stay behind your target when you can.
Don't be an asshole. Being a nice and humble sub-par player is better for your group than being an arrogant ass who happens to be top dps. Take it from the arrogant ass. :)
Be an asset to your group by paying attention to fight mechanics. Stay out of void zones, stop attacking when necessary.
Be nice to your healers and tanks.
I hope this guide makes it easy for new rogues to do reasonable dps without much work or attention. Playing a rogue well can be a blast, so have fun with it!