There are two approximate rules when it comes to this: the easiest, the most illegal and immoral. The hardest, the most legal and moral.
The hardest and most legal/moral way: some very specific information-intensive, labor-intensive fields like real estate development, law, finance or medicine (I knew an ophtamologist who was raking millions a year by doing Lasiks). You need to work hard in school, and then after school.
The easiest and "approximately legal/moral" way: holding cryptocurrency and technology stocks like Amazon for at least 20 years, adding to your stash with every paycheck and without ever selling (except perhaps on obvious overheating).
The easiest and most illegal/immoral way: drugs / opening a darknet market like silk road (ended badly for Ulbricht and Cazes, though: one life in prison, one dead)
The hardest and most illegal/immoral way (not sure why anybody would want to try this): robbing banks, ATMs, jewelry shops
I've personally been lucky to profit from the cryptocurrency train in 2016-2018. I would never have expected to become rich so fast, but here it goes. I would have preferred to rake this money through my own labor, though, because it was honestly gambling.