To use cheats you gotta open your phone, and select the "Extras" option, then choose the "Cheats" selection. Enter one of the following codes to activate the corresponding cheat function. Note:
Achievements and auto save are disabled when cheats are activated. However, you are still allowed to manually save while cheats are enabled.
Be warned that entering some of these cheats in the game will disable certain achievements/trophies, player should do so that their own risk. Accessing secrets is different from cheats, and will not affect the status of gameplay. There is no way to unblock the achievements except for re-loading the game without saving after the cheat has been activated. However, if a player uses any of these cheats, but still desires all the achievements, they can simply start a new game file and complete the first mission. Then there are no blocks on the achievements. Also, because of the way that cheats work, they may have undesired effects on gameplay, such as glitches.
Note that cheats that are found in the DLCs only work in their respective DLCs. They do not work if one switches back to Niko Bellic's storyline, or if you try and use Johnny's cheats, in Luis's storyline, or if you try and use Luis's cheats in Johnny's storyline. However, Niko's cheats work in Johnny's storyline and Luis's storyline.