Find a pain and make it go away. People will pay to have their pains removed. When this really works, get them to talk about it with others sharing the same pain.
You don't have to be the first, or the prettiest, or the cheapest to market with your idea. It just has to solve a basic pain.
To identify that pain, know who has that pain. In other words, get to know your potential customer. Where do they hang out both online and in the real world? What do they spend their discretionary time and money on? Who hangs out with them? That's how you determine how to reach them.
Fail early and fail often. Version 1.0 doesn't have to be perfect. And always keep your eyes open for the next pain coming around the bend.