You need to find a niche/market that works for you. They need to want something you can deliver and reach. Then, what you offer them can vary, either a product or a service.
It could also be an affiliate or CPA offer, which generally also offers services or products. But the difference is that with CPA, some programs pay you per lead generated, which makes a crucial difference:
If you're paid per lead generated, it doesn't matter if the people in your niche have high purchasing power, or if they're stingy or whatever. However, if they have to buy something for you to get paid—which is pretty much all the other scenarios—in addition to wanting what you can deliver and being able to reach them, they need to have purchasing power and enjoy buying things online.
From there, once you have found a niche, you sometimes need to learn things about that niche to better understand your market, to know the niche better than the average customer, in order to be able to find and/or create solutions that they will be dying to get.