I think the approach to deciding on what skill(s) you want to learn would depend on what skills you’re currently lacking or weak in, what your interests are, and what you’re trying to accomplish. Likely from your post you’re trying to learn a skill to increase income.
If that is your goal, tie your interests in with what skills are marketable. Or, maybe there’s a skill you could learn in a current job that could help you to get a raise or a better position.