To get a general overview of its modding mechanics, you may want to look at my Modding Primer.
Check out Jack's thread on the Meshes, it gives an explanation of how things are connected, and should get you started on where you have to look.I'd like to make some more units based off vanilla units. Is there a list somewhere of all the different text stuff I can put in? For instance, 'Armour' says 'cloth.' What else can I put there, etc.?