After buying GTA V, you may start playing online mode. Firstly I would recommend playing until you reach level 15-20 so you can compete in job missions and heists of other players.
There are a lot of cheap appartment, buy one that costs a minimum of 200k (cheaper ones don't offer heists) so you can start heists with other friends.
You can play and prepare those heists with friends or random players. Start by playing the Flecca Heists which gives you fast money for the start. After gathering some money, buy a motorcycle club and buy a Meth or Cocaine factory. Meth and Cocaine give you the most money, money and documents the most less money.
With a friend or randoms helping you, you can get a lot of money in a very quick way. Always sell your products to Los Santos and be carefull, to only buy the cheapest factories.