Creating Units
Also see the documentation for the data model of the Units, the tooltips in the editor and the documentation for RtUnitBhv (the unit at runtime).
Although TBE has already solved many common tasks that you would come across when you would start creating such a system from scratch, making a game is still an intensive undertaking.
Animator and Animations describes how you add Animations to Units in TBE.
Adding your own logic has information on how to instantiate Units programmatically at runtime.