to get a bunch of money SPOILERS you just do heists, which means doing story missions. then (when theyre unlocked) do the franklin assassination missions from lester and listen to his obvious hints in the premission briefing cutscenes. he usually drops lines like 'if you want to make extra money these stocks will go way up if you finish this mission', lol. when he says that take all your heist money, every dime, buy the stocks he says, finish the mission, sell the stocks, profit. the more money you have to buy stocks the bigger the flip, so save all your money, dont buy properties early on.
stocks seem like a scripted event mostly. i had a random blue dot event, some guy needed a ride to the airport or something in 3 minutes, at the end he said 'you wanna get rich, buy tinkle'. id been watching the stocks and the hadnt been moving really. dropped all 3 characters every dime into tinkle stock, then it went soaring as i watched it over time, steadily growing, then it seemed to peak, sold all the stocks, made 7-8 mil each character.