While I could go through the whole run-around of finding various guides and copy/pasting them into here, I will do you all (and myself) a favor and link to important posts to give you ideas on what you can do with your specs/gearing to begin raiding. As always if you have any questions after reading the guides/links below feel free to ask here, we can be a bit more forgiving of a simpler question than those at EJ.
Pretty much everything you need to know for Fire at 85 is here: Fire Mage Compendium at Elitist Jerks
-- It includes stat weighting, basic reforging advice, proper specs/glyphs, and how you should work in your priorities/scorch weaving rotation.
Ditto for frost Frost Mage Discussion at EJ
-- Not quite as elaborate as the fire one, but it includes all of the basics to get you off the ground (stat weights, specs, and casting priorites)
Arcane in Cataclysm at EJ
-- This thread covers pretty much all you need to know about the basics of raiding/instancing as arcane for at least the first tier of content. The stat weights, and the 3-phases of casting [burn/regen/conserve] are all covered.
Get your butt into some normal and heroic 5mans, you won't be stepping into a heroic 5man in leveling gear (you need a gear level average of 329 [visible on your character sheet], most rep rewards start at lvl 333, so get to grinding those out (I was honored with all of the reputations after just leveling through the zones) -- make sure you buy the tabards for the various factions to wear during the instances [you can get them at friendly].
Note that you will need a LOT of hit to be raid capped (1746 if I remember correctly, for 17%) so make note of what the weaker stats are on your gear (this depends on your spec) and reforge those into hit.
*If you are wearing gear with spirit on it -- get rid of it, or at worst case reforge it until you can get rid of it.
Last but not least, use those Justice Points (JPs) that you have been gaining (or kept from WotLK) on gear from your faction's vendor, it is all ilvl 346 with costs from 950-2200 [depending on the piece].
It is important to remember to always read through at least the last few pages of these threads as there is generally some very nice nuggets of information to be found there that may not always be in the first post or updated to the latest patch.
Edited, Dec 14th 2010 8:42am by Anobix
Edited, Dec 14th 2010 10:02am by Anobix