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?)

Good job learning about this stuff. I'd recommend reading "The Little book of common sense investing" by John Bogle. A Random Walk Down Wall Street by Malkiel is also excellent.

Investing should be different from gambling, but there are a lot of trading options that are effectively just gambling. After spending decades thinking and researching this stuff, I strongly believe that Bogle and Malkiel are correct -- your best bet as an individual is to buy the entire market via a low cost index fund/ETF. Don't try to beat the market, as it's too hard to have a sustainable edge over the competition. Spend your time working on obtaining valuable skills and increasing your income potential.

It's interesting to learn about investing in individual stocks, options, futures, financial analysis, but the risk is that you delude yourself into thinking that you can pick the winners and beat the overall market average. Settle for average, and you'll be ahead of most people.

You owe US taxes on worldwide income so that income is also taxed by the IRS. However, you get a dollar-for-dollar credit on taxes paid to Germany so you're probably safe from double-taxation. Read up on the Foreign Tax Credit and Form 1116. Turbotax and its competitors probably handle this. Here's turbotax's blurb on it: https://turbotax.intuit.com/tax-tips/military/filing-irs-form-1116-to-claim-the-foreign-tax-credit/L2ODfqp89