While playing you should press on the CTRL + ~ (that last symbol is a tilde) keys on your keyboard at the same time to bring up the Command Console. After that you can lookup below what cheats there are and what their codes to enter are.

Code	Effect
DebugUnlockPerkByAbility -1	Unlock Perks
DebugUnlockPerkByRequirement -1	Unlock Perks 2
cvarAdd g_inhibitAi 1	Dumb AI
cvarAdd g_permaGodMode -1	Turn off God Mode
cvarAdd g_permaGodMode 1	God Mode
cvarAdd g_permaInfiniteAmmo -1	Turn off Infinite Ammo
cvarAdd g_permaInfiniteAmmo 1	Infinite Ammo