In order to enable the developer console, you must enter the options menu and enter the keyboard section, here you must click the 'advanced' button, a popup will appear with two options, after clicking 'enable developer console', it can be opened and closed by pressing the button above Tab ⇆, known as the "tilde." ~
The first command you will then want to enter into the console is:
this allows commands flagged as cheats to be entered into the console.
impulse 101 - gives you every weapon
god - makes you incvincible
noclip - allows you to float as if there were no gravity
None Playable Characters
Command: npc_create "npc name"
Command: give (weapon_name)
Check out the full Black Mesa Walkthrough for help with the game.