You get $20,000 from Chris. If it's before March, you can buy stock from investment office & wait next month for the stock to raise in value. If it's already March, buy gold bar from jewellery shop. Those gold bars will raise its value until September. Then you can sell those gold bars at pawn shop for cash.
In the future, there should be option to buy drugs from either Red Cat or Hong Family once you completed their quests and sell those drugs to people on the street. So basically you become drug dealers. It's very risky as you can get caught by the police.
Fighting enemies also can be lucrative like previous person has said. Once you get promoted to Elite Arena, you can get $500 if you win against the opponent.
You don't need to become a drug dealer if you can figure out ways to get $60,000. For current version, it's not even possible to become a drug dealer. It's just something that we plan to implement in future version of the game.
