(adsbygoogle = window.adsbygoogle || []).push({}); This doesn't work in public games and I'm sure you can't get achievements when you're doing it.

Go to Options - Keyboard and check the box that says "Enable Developer Console"

On the Menu screen press the squiggle on the keyboard. Type "map " and pick a map from the dropdown list and then press enter.

When ingame, press squiggle again and type this "sv_cheats 1". Then you can enter some codes to do anything basically.

Examples:
-MarineInvuln (Godmode)
-Ghost (Walk through Walls)
-Firstperson (Play in First Person)
-Thirdperson (If 1st Person is enabled, it reverts it to the original camera view)

I haven't found the ones to spawn aliens, objects or NPC's or the one for each gun and infinite ammo