1. I would start saving every bit of money that I could, instead of spending it all.
2. I would develop my art and writing skills early, and seek to do that as a career. (I came close to entering the field of comic book art, but there were too many hurdles for a JW to overcome to work in that field, so I gave it up. Still regret it to some degree.)
3. Once the opportunity came, I'd start investing all of the cash I managed to save.
4. Once I was financially secure, start to enjoy my good fortune... responsibly. I'd move out of the city and to a quiet suburb in the Midwest, and enjoy the nicer people and the serene environments.
I've sort of done those things over the past ~15 years or so, and have mapped out a retirement path that hopefully works out. I just wish I'd started early. Can't do that when you're prepping for the end of the world, though.