This tutorial created by Niektb from Wildfire Games Forum will teach you how to create a new unit using in-game available resources.
Tutorial - Creating new units by Niektb: Trac.wildfiregames.com
Simply follow each step in the tutorial and you will soon learn how easy it is to add new units into the game. Good luck and have fun!
0 A.D. Modding Guide: Trac.wildfiregames.com
0 A.D. Forums (Game Modification Section): Wildfiregames.com