What kind of software? You do different things for Windows desktop app than Mac desktop app than iOS app than a website.

Furthermore, how you do advertising highly depends on what kind of product it is.

Advertising a website for plumbers would be completely different than advertising a website for dentists.

Finally, the methods aren't really that complicated:

* try adwords (and other kinds of advertising networks, like facebook)

* try content oriented marketing (i.e. write about things related to your software and hope someone will notice)

It's just that neither method is a sure way to get attention.