Best Professions for Mage
Picking the profession might be more complicated than just racial selection. For mages Tailoring is the best option, you get Bloodvine Set, Robes of the Archmage and lots of good items (bags at first) while leveling. Unfortunately, Bloodvine Set requires Tailoring skill to equip so you cannot use items otherwise. Tailoring is usually combined with Enchanting, you can disenchant crafted items to get reagents as well as AOE dropped greens and blues.
Engineering is necessary for PVP, you get lots of useful devices and explosives. This profession is not that expensive.
Herbalism and Alchemy is perfect to make good money crafting consumables and sell them at Auction House, besides you always have your own potions, flasks, elixirs for PVP and PVE.