Well I get my miner... send him out to find veins of gold ore.. I then bring that near a forge and smelt it into gold bars, These in turn are then shipped to the mint who make me my gold coins

J/k
(utilizing the GB2)
it all depends on your server, normally a good start is mining/herbalism as mats are always a good thing to sell. but some realms they are saturated so you have to play it smart. Currently, for the next week.. the fastest way to make gold is mine Obsidium Ore, and use JC profession to prospect and cut the gems and vendor them (only good til 4.1 rolls out soon).
Otherwise.. check your realms AH see whats selling, listen i trade chat. But don't be stupid about it and flood the AH with items to sell it will cause issues.
Also try just doing the quests in each of the Cata zones... you'll make some good bank there.. some of the gear you get is worth 20g+, then tehre's there reward gold for the quest.. and the trash gold....
Pick one area and start wit that.. But start with Gathering is my choice, its a guarenteed money maker, just the amount has alot of factors.
But regardles don't expect to go out and buy that 10,000g piece of gear off the AH tomorrow, it ain't happening.