Press Tab, then type the following codes, unless noted otherwise:

Press F2 to enter these cheats:

On the first level, Lambert will ask you to look at the lights first to the left, then right, then above you, then below you. After that, go to your left, where you will notice a platform you can climb. So do a double-jump, then climb it. Enter the door. The room would be dark, so equip your Nightvision Goggles by pressing 2. Look at your left then get the Lock Picks. Then go through all the obstacle. After you jump above the gate to finish the level, NEVER go straight to the door infront of you, but go to your right then pick the lock of the door there. Interact with the computer then get the password '5656'. Go back to the start of the level, and instead of going to the right platform (where you got the lockpicks) go left instead. Do a doublejump then enter the password at the door. You'll be in a room with a woman who's part in training you. You can talk to her, then (if you're bored) knock her out, resulting in mission failure. :)

In v1.1 of Splinter Cell, to activate the console you must press F2 instead of TAB.