Hit the tilde key “~” (you know, that squiggly one left of the 1) to open the console. Type sv cheats 1 to turn cheats on, then enter any of the codes below to get the upper hand. It should say sv cheats set to 1

The only cheat you really need to breeze through the game is Godmode.
Just type god into the console and when it appears under the input field, click on it. Hit Enter (or click Submit), and voilà — you’re invincible! Now you can run through the game without worrying about taking any damage.

No Clipping Mode
noclip

Adjust Gravity
sv_gravity [-999 – 999999]

AI Node Information
impulse 195
impulse 196
impulse 197
impulse 199

All Weapons and Ammo
impulse 101

Auto-reload Disabled
-reload

Auto-reload Enabled
+reload

Control on-Screen Monsters
impulse 109

Delete Monster/NPC from World
impulse 203

Developer Mode Off
developer 0

Developer Mode On
developer 1

HEV Suit, Weapons, and Ammo
impulse 101

List Global Entities
impulse 104

Return to First Person View
firstperson

See Things Brightly Without Flashlight
lambert -1.0001

Set Reload Speed; 0 is fastest, 3500 is default
rate [0-3500]

Strange Colors and Wireframes
r_fullbright 9

Suicide
kill

Texture Name, While Looking at Object
impulse 107

Third Person View
thirdperson

Toggle Game Speed
host_framerate [0,1]

Toggle Light Amplifier
r_fullbright [0,1]