In order to enter the following cheats you will have to activate the console. There are couple of ways to do that:

1) Modify the game shortcut. Right-click on the Half-Life 2 shortcut, select Properties and a new screen should pop up. Add the " -console" parameter (without the quotes) at the end of the target field so it looks similar to this:

"C:\Program Files\Half-Life 2\hl2.exe" -console

Apply the changes and run the game using that shortcut.

2) Open the "autoexec.cfg" file (use Notepad for this) located in cfg subfolder of your game install dir (if there is no such file, just create it). Add the following two lines:

sv_cheats "1"
con_enable "1"

3) Turn the console on in the options menu (Options > Keyboard > Advanced).

Now, use the [~] key while playing to bring up the console and enter the following codes there:

god - God mode (server side only)
give  - Gives a weapon or an item (see the list below)
buddha - Reduces your health
hurtme # - Hurts the player (# is damage amount)
impulse 101 - All weapons
impulse 82 - Spawns a jeep
notarget - Player becomes hidden to NPCs
noclip - Walk through walls (server side only)
maps - Displays map list
map