Ok so you wanna start mapping, well the first thing you do is to install original ET wich you find here:SD ET.
what you do next is to install this in a CUSTOM location where you have all rights, program files on these newer OS's isnt any good anymore.
So go on and install it in ex: C/Games.
When it is done you have the mapping enviroment set up.
Next is the mapping tool wich is gtk radiant. this you will find at icculus.org.
Download the version you want to use, if you choose the latest version you will download a zip file, dont worry it wont be so hard anyhow to install.
unzip the archive into ex. Games again.
now in that folder you will find the radiant exe. start up and it will ask for what game you wanna map to. choose wolfenstein enemy territory, next is where it finds the engine. be sure to set this to where the ET.exe is, NOT etmain.
and voila you should be good to start mapping.
I would suggest to start on this tutorial to get the grip of it:
Its absolutely the best around!