At the start of day 4 you unlock a side quest to go to the bank of South Park. Butters will all you and tell you that he ran out of money and can't pay his minions so you must go with him to the bank to get a loan.
It's a very quick quest. Just go to the bank on main street and use your buddy called 'Toolshed' to blow the lava away from the entrance. Enter and complete the wheel spinning minigame. Exit the bank to finish the quest.
Now to earn the trophy you must go back inside the bank and talk to the guy with the wheel-spinning minigame again. Keep playing this game until you reach the top 0,01% in the stock market. It's all random. You just spin a wheel and hope for it to land on a field that gives you money. If you land on the same field twice you go bankrupt. You also get bonuses from each type of field, such as multipliers or the ability to evade the next bankruptcy. You have to rank up your economic value in this minigame multiple times. When you reach the top 0,01% the trophy unlocks.
To clarify: this has nothing to do with how much money you own in-game. It's all based on that random minigame inside the bank.
