Cheat: Console Mode
To get console mode, right click on the Half-Life desktop shortcut. In Properties, go to the target tab. Look for the line that reads: X:\Sierra\Half-Life\hl.exe.
Enter: "-console" at the end of that line so it reads: X:\Sierra\Half-Life\hl.exe -console. Note the space after ".exe". Now launch the game and notice that the startup screen has a "console" option. There are a ton of cheats you can enter
If you just want Invincibility, enter: /god in the console Mode. If you want Invisibility, enter: notarget in the console Mode.