Fortunately for this particular game, Grand Theft Auto IV, the cheats do not involve any button combinations, but all you have to do it type the digits following into your mobile phone and ring it. Then, which is also good, it stores the cheats in a section on your mobile after all the Phonebook etc. so you don't have to type them in again.
Annihilator (Armed Helicopter) - 359-555-0100
Cognoscenti (Car) - 227-555-0142
Comet (Sports Car) - 227-555-0175
Buffalo (FIB) - 227-555-0100
(Boat) Jetmax - 938-555-0100
(Motorcycle) NRG-900 - 625-555-0100
(Car) Sanchez - 625-555-0150
(Car) Super GT - 227-555-0147
(Car) Turismo - 227-555-0147
THE FOLLOWING IS COPIED FROM THE IGN WEBSITE.
Head to the internet café (or wherever you can access the internet) and insert the URL www.whattheydonotwantyoutoknow.com. If inserted properly, you'll be brought to an in-game message board containing maps of secret locations in the game. Check it out!
Cheat: Change Weather & Time of Day
To randomly change the weather and time of day, break out your cell phone and insert the following number: (468)-555-0100.
Cheat: Weapon Batch 1
To unlock a baseball bat, grenades, M4, MP5, pistol, RPG and sniper rifle, insert the following number into your cell phone: (486)-555-0100. Keep in mind that this will disable your ability to acquire some achievements.
Cheat: Weapon Batch 2
To unlock an AK-47, knife, Molotov Cocktails, pistol, RPG, sniper rifle and Uzi, insert the following number into your cell phone: (486)-555-0150. Keep in mind that this will disable your ability to acquire some achievements.
Cheat: Adjust Your Wanted Level
To get rid of Niko's wanted level, insert the following number into your cell phone: (267)-555-0100. To add stars to Niko's wanted level, insert this number instead: (267)-555-0150. Keep in mind that this will disable your ability to acquire some achievements.
Cheat: Refill Health, Armor & Ammo
To refill Niko's health and to give him some fresh armor, insert the following number into your cell phone: (362)-555-0100. If you want to give him health, fresh armor and refill all ammunition, insert this number instead: (482)-555-0100. Keep in mind that this will disable your ability to acquire some achievements.
Unlockable: Statue of Happiness Shirt
Head over to the Statue of Happiness and climb up as far as you can go. Head around the left side of the statue from where you entered. When you do this, the game will pause briefly and load a new shirt that your character will be wearing, a nice souvenir for visiting the statue!
Unlockable: GTA3 Outfit
At one point in the game, you're given an option to kill Dwayne or Playboy X, and you must choose one or the other. If you kill Playboy X, you can use his loft and change clothes there. Amongst your change of clothes will be the outfit from GTA3 -- a tight black jacket and green cargo pants.