From there you can download lots of maps, simple download any of them, open them with Winrar, or Winzip. (unpacking programs). Put them in your "steam/steamapps/yourusername/half-life/ESF/Maps" folder. And you can find the game in the map list when you create a server.
Just a note, some map includes custom *.WAD, Model, Sprite Files. If they do, simple put the files in the correct folder:
Model (*.MDL) files in "steam/steamapps/yourusername/half-life/ESF/Models"
Sprites (*.SPR) files in "steam/steamapps/yourusername/half-life/ESF/Sprites"
Texture (*.WAD) files in "steam/steamapps/yourusername/half-life/ESF/"
Map (*.BSP) files in "steam/steamapps/yourusername/half-life/ESF/Maps"
Just a note, some zip/rar files may also already include the correct folders. For example, a zip/rar file may have this: