The four currently available level 80 raid instances are Naxxramas, Obsidian Sanctum, the Vault of Archavon and the Eye of Eternity. Each can be attempted on 10-man or 25-man ("heroic") difficulty. You can walk right into Naxx and Obsidian Sanctum as a fresh 80, but to face Malygos in the Eye of Eternity you need to have killed Sapphiron for the key. Archavon is linked to victory in Wintergrasp and drops PvP as well as PvE loot.
Naxx is likely your first stop - the earlier bosses are more forgiving and there is a lot more to be gained from an instance with multiple bosses over the three single-boss raids. To enter Naxxramas, fly to where it's marked on the Dragonblight map and enter the citadel from underneath. There's nowhere to repair in the instance, but if you die and release you are ported alive to the nearest graveyard (much like Tempest Keep) so you can detour via your faction base or summon a mammoth mount in the graveyard.
The Obsidian Sanctum is underneath Wyrmrest Temple -- the only portal with a green swirly. The Eye of Eternity is located in the Nexus, higher up than the 5-man entrance, and the easiest way to get to Archavon is via your faction's portal to Wintergrasp from Dalaran.
Before you enter: Gear
If you're already in full Sunwell gear, you might be wondering if you even need to bother finding gear upgrades before venturing into Naxx. To some extent, your lassitude is rewarded - for most classes, Sunwell gear will do pretty well. However, things have changed - you'll need a lot more hit rating to cause pain to level 83 mobs, for example. Tanks in particular might find they take a particularly hard beating without any WotLK gear, being well below the crit immunity cap. However, there are plenty of items available through crafting, quests and 5-man instances that can complement Outland gear.
Don't underestimate the value of stat points gained through gems and enchants, too. It's not worth regemming every single piece of gear you have with Northrend blue gems, but jewelcrafters and enchanters skilling up can effectively give you free (or cheap) stats. Leatherworkers and tailors might have blue patches going spare, though these are likely to be more valuable and probably worth saving for new upgrades.Specs and Glyphs
Don't forget your spec. Turning up in your levelling spec might not exactly be optimal for a raid, so read up on your class and decide your PvE route. Dual specs aren't in yet, so be prepared to fork out the cash if you'll be doing double duty. If you changed glyphs for levelling, sort those out now as well, and remember you got an extra glyph slot at 80.Strategies
If you've done Naxxramas at 60 or 70, or you were a beta tester, you're probably pretty familiar with most of the bosses and how to defeat them. If not, there's plenty of information already out about Wrath's initial instances. Trying to learn every boss at once on-the-fly is a surefire route to a headache, so do a little bit of homework and know your Patchwerk from your Sapphiron.
None of the strategies involved in Naxx are particularly taxing once you have the hang of them, although mechanics like Heigan's dance, Thaddius's polarity switches and Sapphiron's ice blocks will likely catch those new to the encounter unawares. All three of these are a lot easier to learn by doing than by reading about them, so don't worry if you make a mistake the first time -- but do your best to learn from it!Consumables
Still got tons of raid consumables left from level 70? If so, you can use them as a base for your brand new level 80 raid bag. Note that you can't use weapon oils on any weapons you might have picked up post-70, so you can throw them away. Northrend food is easy enough to find; several recipes are purchased with Cooking Awards (tokens gained from the cooking daily) so co-ordinate with other raiders to make sure you don't all buy the same one first.
Flasks and elixirs are dependent on a friendly alchemist -- maybe yourself -- skilling up, and frost lotuses, the Northrend version of fel lotus, aren't too easy to come by just yet. It's unlikely you'll be able to walk into your first Wrath raid with a bagful of brand new pots, unless you're very rich or spend all your time farming herbs; the old stuff should do to start with. Bear in mind, though, that the Shattrath soulbound flasks don't cover Northrend instances.
Health and mana pots have been limited to one per fight since 3.0.2, but you should have run across a handful of these while levelling (especially if you did the Ampitheater of Anguish) and they're not too hard to come by otherwise.Priorities as a new level 80
When you ding that final ding, and no longer have the experience bar as your goal in life, there are a huge range of things you can do. From dailies to raiding and tradeskills, what should you do first?
There's obviously no clear-cut answer, but there are a few things from a raiding point of view that take precedence. If you're after server first 25-man achievements, and especially if there's another guild close on your heels, your priorities are probably to help people level and then to jump into raids at the earliest possible moment. Although I've mostly talked about Naxx above, the Obsidian Sanctum is clearable with a newly-80, half-empty raid of people in Sunwell gear -- as long as you kill the three drakes around the sides, which will otherwise land during the fight and make your lives a misery.
Your own tradeskills might end up being fairly important, as you can craft immediate gear upgrades that will make things easier in raids. You can also start running 5-mans and heroics with other 80s to get upgrades and reputation. Plus, there are dailies for factions such as the Sons of Hodir -- who provide shoulder enchants -- that you'll want to start doing as soon as possible in order to make the most of all the shiny new gear you'll be getting.
Talking of shiny new things, if yo're loaded with cash (or have a guild bank sponsoring you), the Traveler's Tundra Mammoth
is a good buy for repairing during raids, plus you might want to blow 8k on one of the Dalaran rings, sold in the jewelcrafter's shop.
Of course, it's also appealing to fly around, getting the explorer achievement, and then fish in Dalaran sewer for a few hours. You've certainly deserved a break, but if you're pushing to clear content before anyone else on the server, sadly you can't wait forever.
Good luck in Northrend!