What skill has the highest chance of enabling you to be self-employed?

If you are starting from zero, what skill (technical or non-technical) would give someone the highest chance of becoming self-employed in the shortest time period (e.g. doesn't require 10 years of training to even get started)?

Looking for skills obviously that maximise income, flexibility and location independence.

Of course programming is an obvious answer - but can we be more specific? :)