All right, welcome to the world of mapping. First, beware because many versions of the Radiant and of the ufo2maps available for download are corrupt. Try to download another version of the UFORadiant, maybe the one from the 2.5dev. I am including the ufo2map.exe, the tool you will need to test and compile the maps. Run it via DOS, the procedure is:
ufo2map -fix maps/[path of the map]/[name] This will fix some errors
ufo2map -check maps/[path of the map]/[name] This will show you all errors of the map. You will use this a lot
ufo2map maps/maps/[path of the map]/[name] This will compile the .map file into a .bsp readable by the game
To start with the mapping, follow the "house" tutorials although they refer to the older version of the Radiant they are good. Then, open a small map tile and play with it a little. Good luck boy, the first time I was almost crying and I use AUTOCAD at work...