There are many complicated ways to do that.
On the Internet, however, it is usually through specific demands. You find a specific need or want or problem, try to come up with something that fills, satisfies or solves, then consider that demand group as a market segment or niche.
The reason for this is that Internet marketing is basically search engine marketing, and search engines work with keywords. These keywords or search queries are almost often related to some need, want or problem.

