What do I need?
You need a mapping program called gtk Radiant.
You can download it here: Gtk Radiant 1.5
In my oppinion is Radiant 1.5 the easiest to learn mapping. Many mappers still prefer Gtk Radiant 1.4.
If you want Gtk Radiant 1.4, download it here: [url=http://www.herrks-base.de/downloads/GtkRadiantSetup-1.4.0-Q3R