I'd say there isn't really a "one shoe fits all" for marketing online, but a good starting point would be to work out what problem your application solves for people, establish who those people are, and then what channels you can use to market to them effectively.
And also just be open to testing things out, go against the grain if you think it will bring results, and be genuine and engaging with your content and communication.
This last point is really important I think.
The internet already has plenty of marketers, and consumers are wary about being sold to.
In most cases you really have to earn the right to pitch your product to someone.
