To download maps is rather easy to do, go here first:
http://www.esf-world.com/main.php?content=download&category_id=1097
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:
----
Models <-- folder
Sprites <-- folder
Maps <-- folder
texture.WAD
Readme.txt
-----
If you have something like this, simple extract all in your ESF folder, and its done.