http://forums.legionsoverdrive.com/threads/dts-tutorial-process.3643/
This tutorial link above is exporting from maya , there are exporters for Maya , Blender ( Free ), Max , Softimage available from GarageGames. Theres bajillions of tutorials for learning how to model/texture for Blender. If you get confused on the export process maybe wuzgud can help you , he managed to export from Blender. ( The one above is for maya , the steps will be similar but in different places )
or you can try constructor
http://www.garagegames.com/products/constructor (Free)
I suggest if this is something you might want to pursue in life you choose from the top 4 ( Probably Blender ) , they are vertex modeling where as constructor is its own wacky thing that will never really apply outside of Legions, HL 1/2 and Quake and not into the future.