Hi, try this:

Add -console to the end of your Half-Life command line. Or, with the 1.0.0.9 or above patch, just run the shortcut called "Half-Life Console".
Then, to get to the console, press the ~ key (or the key left of the "1" key for non-US keyboards).
If that doesn't work, you've probably bound ~ to something else. To fix this, you have to go to configuration => controls and click "Use Defaults".

Also, or after the above, depending on what you want to 'add':

Enter the console and type sv_cheats 1

May help