I think that depends entirely on your interests, existing skills, and location. What skills are in demand locally (or do you anticipate relocating?). What do you have aptitude in?

If you like to be hands-on, learn trade-related skills like welding, soldering, and/or plumbing. With a little more math background, electrician skills are good to pursue.

If you're more academic, get the skills that fewer people have (which often means STEM). Learn to communicate well, learn people skills (Dale Carnegie's "How to win friends and influence people" is surprisingly relevant and excellent).

It's really hard to become the best in the city/state/country at something. It's much easier and lucrative to get really good at a few things and then be one of the few people who possess that niche set of skills.

At your age, one of the more attainable skills is to learn how to use a spreadsheet really well. See youtube/coursera/etc. Think about how to use it not just to calculate, but to plan projects, track tasks, and to view/slice data (it can be used, for example, to sort sales by date, product type, time of day, location etc -- can you see the possible relevance and value?)