[xTomahawkx]

Hands down - gathering. There a lots of materials on high demand that go for good prices simply because high level gatherers wont go for it anymore. That's a beginning. Later you get a feeling for things that sell well. Watch the marketboard. Highend is aethersand, rare materials from timed nodes and so on. 

But forget the 10 million gil per week nonsense. That's doable on certain servers but not so easy.