If you don't have a steam account, you have to make one. You can either enter your HL cd key, or if you've lost that, you have to buy a new one, the cheapest option to get NS running is to buy base HL for $9.95
Once you have steam installed it will download the half-life files for you, all 393 or so megabytes worth. You'll have to just wait it out, you can't "install half-life into steam" using your original WON cd.
You can also download a steam installer that has half-life and other games included so you don't have to wait for steam to retrieve them from their servers:
http://www.fileplanet.com/130416/130000/fi...che-%5BWin32%5D