http://www.dafront.com/~nemesis/
On this page you will find everything you need to start mapping, I started a few weeks ago with about the same frustrations as you sound to be having. This page will shed some light on the subject.
A word of warning though, map building with Radiant, while fun and creative is still very frustrating. As soon as I fix a problem or figure out how to do something, something else takes it's place.
Still worth it though..
Good luck!
Booter