Enable Game Console
You'll have to edit a game file to enable your console. Relax it's easy, follow the steps below.
First open your Window explorer.
Next locate your Hitman folder located at (by default)C:/programfiles/eidos/lointeractive/hitman
Now, make a copy of the Hitman.ini file BEFORE you edit it. Open the Hitman.ini file with wordpad or notepad.
Add this line of code at the bottom: enableconsole 1
Now close it and start the game.
During the game you will now be able to hit the Tilde (~) button to open the game console. This is where you will be entering the game cheats.

Game Cheats
Enter these codes into the game's console (after it is enabled):

Invulnerability - god 1
Infinite Reloads - infammo
All items (weapons, keys, letters, etc.) -- giveall
Enemies Don't Attack 47 - invisible 1