After you have created the game you can press ~ at any time to activate the "console". If the concole dosn't come up right click on the windows desktop goto new click shortcut. target the short cut to "C:\SIERRA\Half-Life\hl.exe" (note : C may have to be replaced with the name of the drive you installed it to) but also add "-game esf" and "-console" after the shortcut, these are called command lines
your shortcut should be targeted to
C:\SIERRA\Half-Life\hl.exe -game esf -console
then when you press ~ , the console should come up. You don't have to press shift people just say ~ because ` is also another key.
The console can also be used for many other things, try typing
sv_gravity 100
That'll lower gravity.