First, you need to download Steam from the link I provided. Then you need to install it to your hard drive. After you install it, you need to create a Steam account. After you create an account, you need to register the CD-Key that came with your copy of Half-Life to your Steam account. After you register the CD-Key, Steam will download the files needed to play Half-Life off the server and update them to the latest version. After that, you can install ESF.