well I would start somewhere in the tutorials, maybe XML or add a unit tutorials. Which may lead you to more tutorials....
Have I said tutorials enough yet? just kidding, its not all that bad.
you'll probably want to start looking for artwork if you haven't found it already. Mainly speaking, unit models and textures. If you already have that, you may want to consider finding or making a button for your unit(s). There are many bundles of buttons to download here in the downloads section. If your just re-using the old artwork, disregard the preceding paragraph.
Then perhaps the XML. If you've modded other games, you'll probably grow comfortable modding civ4 relatively quickly. The XML files you need to copy and place in your mod (in the same folder structure as Civ4/BtS) are;
CIV4ArtDefines_Unit.xml
CIV4UnitClassInfos.xml
CIV4UnitInfos.xml
If you want to make any of them unique units, change missions or other fancy stuff you'll have to probably edit more files. But most unit changes are just those 3 files.
And always remember to never edit your original files. Create a mod
(C:/.../Sid Meier's Civilization 4/Beyond the Sword/Mods/
), and
copy the files into your mod. Never cut! Always copy!
happy hunting.
Shiggs