Start the game with the parameter -console, eg.

 "...\bshift.exe" -console"

 Before starting the game press [~] to open the console and type in

 sv_cheats 1

 Now start the game and you have the following (widely known from earlier HL releases)
 codes:

 god                  God mode
 noclip               Walk or fly through walls
 impulse 101          All weapons
 sv_gravity #         Set gravity (# = 800 is normal)
 sv_accelerate #      Indirectly set jump abilities by tuning the air acceleration (# = 10 is default)
 skill #              Set skill in-game, 1 = easiest, 3 = hardest.
 map x"               Jump to map x, where x might be:

                      ba_canal1
                      ba_canal1b
                      ba_canal2
                      ba_canal3
                      ba_elevator
                      ba_hazard1
                      ba_hazard2
                      ba_hazard3
                      ba_hazard4
                      ba_hazard5
                      ba_hazard6
                      ba_maint
                      ba_outro
                      ba_power1
                      ba_power2
                      ba_security1
                      ba_security2
                      ba_teleport1
                      ba_teleport2
                      ba_tram1
                      ba_tram2
                      ba_tram3
                      ba_xen1
                      ba_xen2
                      ba_xen3
                      ba_xen4
                      ba_xen5
                      ba_xen6
                      ba_yard1
                      ba_yard2
                      ba_yard3
                      ba_yard3a
                      ba_yard3b
                      ba_yard4
                      ba_yard4a
                      ba_yard5
                      ba_yard5a