Well, first you should play a little with the Deus Ex SDK editor. Get it from http://www.fileplanet.com/files/50000/50191.shtml You will also have to read tutorials. These will guide you step by step how to do things in the editor
sites:
http://www.planetdeusex.com/constructor/tutorials.htm
http://tack.planetdeusex.gamespy.com/
For now, those are the things you need when you start playing with the editor.