To make a texture emit light, you need to create an info_texlights entity.
Then you need to go into it's properties and turn off SmartEdit. After that you need to create a new key.
The name of your new key must be the name of the texture you need to emit light. In the "value" field you need to specify the amount of Red, Green, and Blue color (with the cap of 255) and the brightness.
There are some other methods, but this is the most newbie-friendly one.
Also, I suggest you to read ALL the Half-Life mapping tutorials on this site. They are really useful.
And check
this thread.