I would have tried harder
a) to get better practical skills
b) to get better social skills
c) to become an actual expert in something rather than only having a broad pool of shallow knowledge.
I'd still keep the broad pool of knowledge, but I spent an awful lot of time as a teenager reading fanfiction, and I could definitely have been spending that time gaining more marketable skills, but at the time I trusted that because I was acing my classes, I would have the right set of knowledge later to be valuable without great social skills.
(Spoiler alert: acing secondary school classes is not actually a particularly marketable skill, except when marketing yourself to universities.)
Also, I'd have possibly tried to get some money-earning side hustle started back then.