Some version of this question is one of the most common by instructors so I thought I would write this, which is a summary of advice from many experienced instructors.
I have compiled a list of things you can or should do to market your course. None of these are short term or an instant path to riches. But, they are things that work if you are serious about building an online business on Udemy. Think strategically, not short term. It often takes a few years of effort to begin to make a significant return on your efforts.
The above is only my advice, but informed by the experience of many other successful instructors,
@MassimilianoAlf: This is a great list with a lot of value... every new instructor should take the time to study it, thanks for posting it!