By playing the game. But if you want to level up faster, you need to purchase a little of GP and then spend them on boosters and girls. Also, play Thunder Alley with them too, you will level up even faster.
What's the best online job that gets you money for real? I have tried selling my game accounts on steam, but it is not enough. I used to sell WOW accounts that were fully upgraded for some crazy money before, but now it is not as popular. I have read about ideas to make money, but I am not sure if it really works. They have an idea to become an Uber driver, does this pay good? I have to find a way to make money, I even thought about creating a gaming YouTube channel because I am playing games that much already.