That's a tough question to answer without a little scope. Are you looking for a leveling guide? end-game raiding guide? reputations? theory-crafting? role-playing? questing? power leveling? pvp mastery? farming gold (as a mage)? There are so many different directions guides can go and even multiple medias (some pretty awesome raiding videos out there these days).
I of course am biased towards using WoWWiki as a starting point.
- Guides - Guide directory
- Mage PvE Guide - May be outdated for end-game, but probably still has lots of useful tips and tricks
- Mage PvP Guide - Same thing, but for PvP
- Commercial guides for WoW - This includes general guides as well as class specific
There's a couple portal pages that also might get you started on your "quest:"