All cheats should be entered during normal gameplay using Niko's cellphone. Press (Xbox 360) or (PS3) to bring up Niko's cellphone, and press it again to view the numbered b*ttons. You can now navigate these b*ttons using the directional keypad. Find the cheat you are looking for from the table below, and dial the corresponding number into Niko's cellphone.
WARNING: It is recommended that you do not save your game after using cheats. Please use them at your own risk!

Restore Niko's health. 4825550100
Give Niko armour. 3625550100
Weapons 1
Baseball Bat, Handgun, Shotgun, MP-10, M4, Sniper Rifle, RPG, Grenades enter
4865550100.
Weapons 2
Knife, Molotovs, Handgun, Shotgun, Uzi, AK47, Sniper Rifle, RPG enter 4865550150.

Remove Niko's wanted level. Blocks achievement Walked free. Enter
2675550100
Add one star to Niko's wanted level enter
2675550150.
Change Weather/Brightness
Alters the weather/brightness of the game enter
4685550100
S an Annihilator in front of Niko enter
3595550100.
S a Jetmax in front of Niko enter
9385550100.
S an NRG-900 in front of Niko enter
6255550100.
S a Sanchez in front of Niko enter
6255550150.
S an FIB Buffalo in front of Niko enter
2275550100.
S a Comet in front of Niko enter
2275550175.
S a Turismo in front of Niko enter
2275550147.
S a Cognoscenti in front of Niko enter
2275550142.
S a SuperGT in front of Niko enter
2275550168.