Once you're out in Los Santos, there are many options available when it comes to making money fast in GTA Online.
Players with fresh characters can also try the GTA Online Career Builder, which grants a few million dollars to get you started. Obviously, you could just go out and buy a GTA Online Shark Card with your real money to bolster your in-game bank account with more GTA dollars, but you shouldn't need to spend real money if you know the best ways to earn it in-game.