Grinding is the best way, however, Rockstar has replaced the original kind of grinding with a new style. It's called "IRL grinding" it's really simple. You work your ass off all day at a real job then you come home and give them all your money in microtransactions so you can actually afford their ingame content. It's all the rage.
Screw that. I'd rather work hours for money in an online video game than work hours for real life money to SPEND IT on an online game. I have a job, but I don't want to spend all my hard earned money on an over-hyped online & game which R* sold 1 billion copies of, but that's still not enough money for them...
I'm not hating on the game by the way, otherwise I wouldn't have spent £38 on it, it's just over-hyped and there way of trying to make more money is laughable.
If you only have 8k youre obviously not a very high level. Just play the game, do some missions or races. It comes as you progress.
I am not a high level, but I lost so much money due to little kids killing me or spectating the missions I do, wait till I'm done, then kill me. I'm only level 12 atm LOL I haven't really been bashing it out much, but I've been having fun.
Thanks for the tips, everyone!