http://dynamic4.gamespy.com/~mohrad/
under the files section you need the map editor called MoHRadiant (also referred to here as the SDK)
under the tutorials section you will find guides on how to start making maps with the editor and there are links to the other tutorial sites.
start by finding the ones that tell you how to setup the program... either at this site or linked to Nemesis' website.
you will need to practice making small test maps firstly like the "my first room" tutorials suggest so that you can understand how everything works and how to get a map to functioning state before you start making a real map.
HTH
r3mix