Most money in the game get earned by smuggling booze & narcotics, this is a job which requires a lot of training or else you spend more time in jail then you see your own mother.
It is recommend to begin trying this from the very start of the game.
How do you do that?
Go to the “Smuggling” page (crime on the menu), on this page you see how many units you can carry. Select this many units of booze/narcotics with the lowest price and select buy, type in the code and press on the Buy/Sell button.
Repeat this till you succeed into buying the units.
Then “Travel” to a nearby town where you can sell the booze or narcs for more money – and sell your stash at a profit!
Repeat this while you are doing crimes and stealing cars.