Start by doing the special scavenger hunts that are available in free mode-Bounty Targets for Maude, the revolver Treasure Hunt and Los Santos Slasher-for a total of $250,00 each.Search GTA wiki for the names of anything mentioned below to get more info.
The key to making money now is the Cayo Perico heist, so for new players it's all about accumulating the $2.2 million needed to buy the Kosatka submarine that unlocks it.