What u want to do is go to "settings-keyboard/mouse" and make sure developer console is ON, (be aware this is how I do it for PC). Go back to the main menu and test it out in single player challenge, (it is recommended to use challenge mode instead normal because the game recognizes the cheats and erases the progress. Besides, where's the challenge in that?). After it's booted, press "~" button under the esc button and a code menu will show. Toggle this door pressing it again. Use the code" sv_cheats 1" to turn them ON, (1 and 0 are on and off. Sometimes just using the cheat again turns it off). Here are some cheats u can do:
ent_create_ -make stuff like: weighted_(companion)_cube/antique/sphere
sv_portal_placement_never_fail/bump -place portals on any surface and/or portal, (portal in portal)
sv_gravity XXX -change gravity, (XXX stands for numbers)
fire_energy/rocket_ball/projectile -make rockets and energy balls
noclip -fly through stuff
god -INVINCIBLE!

Those are ones I use meer often. For meer and help, check out this:link

P.S. this perfectly veilig and some u find might not work.