So you're level 80!  Congratulations.  You have survived the grind where the XP bar is your best friend and your worst enemy.  But the fun is nowhere NEAR over!  You cannot just jump into a raid now and expect to top the DPS meters.  Let's look at a few important things you need to take care of before you launch yourself into World of Raidcraft:
  • How's your Spec?  Unfortunately, Blizzard has not seen fit to give mages two viable raid specs.  Other classes are lucky to have them if they do.  The flavor of the expansion for mages is quite unanimously Arcane.  It's got an easy rotation and huge amounts of burst damage.  The AOE is behind compared to other specs and classes, but the single target damage is right on top.  I know there are people that swear by fire or frost and some people that can even make it work with good gear and a lot of effort, but most mages go for the Arcane.  I'm a Frost junkie myself, but I still stick to Arcane for raids.  As for your spec, it's a general rule of thumb it's never a good idea to have all 71 points in one tree.  There's goodies in the lower parts of the other trees that will help your damage.  Most Arcane mages dip into the Frost Tree for Elemental Precision (hit assistance) as well as Icy Veins (cooldown.)  They usually take a few points in Fire as well for Incineration (crit bonus).  You by no means have to stick to a cookie cutter spec, but here's a general spec to start you on your way.  There's some extra points to play with in other talents as you see fit.
  • Is your Gear viable?  Assuming you stick to Arcane, there are specific stats you will stack and others you just let be.  Like every DPS class, hit is your number one stat till you're capped.  There are two talents that can help you out here:  Precision in the Frost Tier and Arcane Focus in the Arcane tree.  446 is the magic rating number if you have no talent or class assistance.  Here's a nice spell hit explanation to help you figure out what your magic number lies.  What other stats do you look at after this?  Most guides rate haste and spell power viable at the top of the desired list for Arcane mages.  There is a soft cap on haste, but it's pretty high (1534 to be exact); chances are as a new mage, you won't run into it.  After those two stats, Crit rating comes in third followed by Int and Spirit.  Intellect gives a crit bonus, more mana, and with the right talent, spell power.  Spirit gives crit rating, but is otherwise useless.  As for gear types, it's probably not a good idea to start raiding in greens and blues.  Blizzard has made epics easy to acquire through the badge system.  It's pretty nice now that you can start getting Triumph badges by running random normal instances through the LFG system.  With luck you may well have enough for a piece or two of tier gear by 80.  On top of that, you'll pick up some nice loot along the way in the instances until you can hit up heroics.
  • Have you Gemmed and Enchanted?  Yes, this stuff is expensive, especially the new epic gems, but if you plan on raiding this is IMPORTANT.  You cannot justify raiding without being fully enchanted and gemmed because it shows that you're not willing to put for the time and effort to help the raid.  With the new ability to disenchant things in instances you can start acquiring materials as soon as you hit Northrend.  On top of that you might find that enchanters in your guild or even strangers will do the enchants for free if it gives them a skill up.  It doesn't hurt to ask around.  Badges and Honor can be exchanged for uncut gems and you can get them cut for cheap or free if you know someone.  With epic gems, it's usually a good idea to ignore the gemming bonuses.  The only time I gem for bonuses is if there's a gem required for the meta activation.  Straight spell power is your #1 gem.  The only viable meta is the Chaotic Skyflare Diamond.  For that meta you need 2 blues, so throw in a couple SP/Spirit gems where they give you a bonus.  As a general rule of thumb, enchant for spell power as well.  Some gear slots give you only options of haste or crit enchants (cloak and boots) and those are both alright if nothing else applies.  Make sure you see what bonus you can get from your professions and use it as well.  Black Magic is considered the best 1 Hander weapon enchant right now, but some people still choose to use Spellpower.
  • Do you know your rotation?  Assuming Arcane again, the general rotation involves stacking Arcane Blast up to 4 and hitting missiles AS LONG as you have a Missile Barrage proc.  You can keep throwing out AB till you get that proc, but I've heard of instances when you go quite a while till it comes up.  Usually for me it pops up in the middle of my 4 stacks.  I'll sometimes throw out a couple more ABs after the 4 stack for the high crit damage so long as my mana's not desolate.  If it's a movement intensive fight you can throw out an Arcane Barrage before you have to move to use up your stacks and still get the damage, especially if you're going to be running around or away long enough that the AB stacks will wear off.  If you have a situation where AOE is the best option, Blizzard is your best bet.  I usually throw off a Flame Strike before my Blizzards for the higher damage then keep spamming Blizzard afterwards.  Your always going to have better damage if you burn your cooldowns before these.  I save mine for important fights - bosses and big trash pulls.  And don't forget about Evocation, mana gems, and potions for when your mana bar is looking low.  There's also the two defensive cooldowns if you're pulling agro, Iceblock and Invisibility.
  • Do you have consumables?  While most raids have people that bring Fish Feasts along for the food buff, I usually carry around some Tender Shoveltusk Steaks just in case.  In the case of PUGs or even heroics, it doesn't hurt to be ready for anything.  Health and Mana potions are important in a fix and I like Potions of Speed to burn with my cooldowns, but keep in mind you can only use ONE potion the entire time you're in combat for an encounter.  If you're raiding, you're also responsible for supplying your own flasks.  Flask of the Frost Wyrm is really the only one you should be using for raiding.  Flask of the North for alchemists is fine for heroics, but 125>47, so stock up on the Frost Wyrms. 
  • How does loot work?  For most PUGs, loot is handed out on a high roll basis, usually with one item per person.  However, some guilds have unique rules for loot and anyone that runs with them should be familiar with them.  My guild, for example, uses a reverse roll sort of DKP and lowest number roll always wins.  If you're going to be a regular raider with a group, know their loot rules and keep an eye on your DKP (or whatever) however they track it.  Usually one person is responsible for keeping track of this on a website or in a mod, but humans and machines are fallible.  You're responsible for taking care of yourself.
  • Don't be a Jerk.  This is simple enough.  While snide and snarky comments are acceptable in some guilds, it's generally not a good idea to be the loudmouth that's spamming meters or calling people nubs over vent.  Don't whine if you don't win the loot you wanted or cry if you die in an encounter.  Raiding is about being part of a team, even if you don't like everyone one in the group.
With the new LFG system and the cross realm heroics, it's a heck of a lot easier to get a group, but DPS still has to wait longer than tanks or healers.  Mages don't have the advantage of being a hybrid so all you can queue for is DPS, even if you're really quick with bandages.  Chances are you're going to meet your fair share of asshats and jerks, as well as find some people who are really great to group with.  I usually try and get my kicks in with people in my guild and it's an extra bonus if you can get a tank's queue or even a full group.  I've found the best way to get through the bad groups is to just do your job to your best ability and push through.  Keep in mind that DPS is in such excess that once you leave a group, your spot will be filled immediately and you'll be branded a jerk.  Despite that, if a group is not performing or full of jerks, it may serve you well to get out of there ASAP and just requeue.  When it comes to random heroic loot, my rule of thumb is Need what you Need.  And I don't mean what you need to burn for enchanting mats or what you need for your alt or even a tank trinket that might be useful in PVP.   If you're on your best behavior, chances are you won't have as many issues.  There will always be asshats and ninjas out there, and some people assume the worst.  Just be smart and take care of yourself and you'll find you enjoy the game and raiding more.