Find an itch, then scratch it. If many people have the same itch and can use your solution, you win.

Simple as that.

The converse: Most itches will either be idiosyncratic, and not get you much attention, or lots of people will be scratching them and it's hard to come out "on top".

I scratch lots of itches, but I also know that most of them are very, very fringe. So going into scratching itches expecting fame is not going to go well for most. But scratching itches is satisfying, so for my part at least I don't care.