I've been digging around trying to solve this puzzle for a bit too. So, I'll probably give you the 2nd serious answer you've received, and that is: SALES. While that's a wildly generalized answer, for me, it appears to be the basic answer. What are you good at? If you understand how to SELL that thing to people, then go for the gold. I tend to be good at understanding investments in equities and insurance contracts, but I'm tired of the structured, 8-5 crap that comes from that industry. I'd prefer to "free-lance", if you will, and sell product to people independently. I still have to have my licensing in place (done), and I'm still required to be uphold some standards and reporting, ie, not defraud people or tell them un-truths, as so many in the industry have done. Coincidentally, those people are in jail now.
So, what do have a knowledge in? Whatever that is, find the SALES angle on it. If it's stocks, then do like me and sell independently. No salary, no benefits, but all cash. Lots of it. If it's cars you're good at, you gotta sell cars or parts, or services related to it.
As a bonus, I'll tell you what WON'T make you money, fast: being someone's "employee", working in the "service" industry, working at a place that requires an 8-5 shift, anything that doesn't require at least a diploma, buying something first then reselling it later, pyramid schemes where someone else is getting a piece of your action.