If I were graduating high-school, or getting out of the service, or looking to change jobs, and assuming I still had a knack for programming in those scenarios, or the ability to pick up such a knack, I would probably dive straight into Kotlin/Java and Android development. If I listened to a future self. Or a parallel universe self.
iOS (iPhone) and Android are each very clean stacks with one tool, one language, one target. iOS used to be the ticket but Android is now (and for some time has been) the more lucrative market.
There is demand and infrastructure for work remotely for Android developers, and I would definitely target a digital nomad lifestyle for geographical independence. This tact is probably the easiest to break into from zero experience, the fastest skill to acquire and also has entrepreneurial leverage as you could not only work contract, but also make and market your own product.
The second contender…
If you don’t like programming, learn enough WordPress to make a landing page. Learn persuasive design and copy-writing. Learn how to manage a Facebook ad campaign and track results and then hookup with aliexpress, the amazon of drop shippers, and start pushing product with ads. Learn from your mistakes and successes and build until you’ve achieved a comfortable minimalist lifestyle with enough left over to invest for early retirement. Also geographically independent.
The third contender…
Possibly the riskiest but most exciting, would be to be a modern Edison, be a maker. Learn how to solder, how to work with micro controllers like the Arduino and Raspberry Pi, learn how to wire up sensors and servos to make intelligent things. It’s dirt easy and cheap for anyone to get started. Be able to build versatile interactions in the physical world, sentries, toys, simple robotics and automation, plant waterers, geo perimeters, security systems, drones.
Learn how to take a fun prototype all the way through fabrication. Borrow or save money to visit Shenzhen and make contacts and setup process. Take some of your prototypes there all the way through to production and setup an inventory on amazon for prime delivery. Push product via Facebook ads.
To take it to the next level: learn Blender and how to design things in 3D. Learn how to use services like shapeways to print your design in the physical and use these as casings and housings for your electronics, as parts for your drones. As art pieces, games and cool trinkets.
Last but not least contender…
With PWA you can leverage sophisticated media capabilities of HTML5, auto-updates, and piggy-back on the increasing access to local sensors on mobile devices and the full spectrum of services in the cloud. You can register and unlock paid content and capability with standard web payment systems giving you standalone applications that circumvent app stores, sprawling web deployments and infrastructure overhead.
There has never been a better time for the autodidactic entrepreneur.