To activate cheats, during gameplay you should press on the ~ (that symbol is a tilde) key on your keyboard to bring up the Command Console. After that you can lookup below what cheats there are & what their codes to enter are.
Cheat Code Effect
God — God Mode Code
G_fov xxx — FOV Change; xxx is the desired number for Field Of View
com_showfps 3 — Show FPS; shows the Frames Per Second on-screen
Listcvars — Codes List; brings up all the cvar commands, but only God Mode works in the consumer version of the game
ListCmds — Commands List
/timescale x.x — Game Speed Change; replace x’s with numbers, 1.0 is default game speed, 0.8 is 20% slower than default, 1.2 is 20% faster than default