so just to add my $0.02 - Kael's guide is a must read, but don't use his examples as they are way too out of date. modding basics is a better guide.

The best tool I have found learn the format of modding, is to take someone else's mod, and open the .xml (or .sql if you prefer that method (completely different)) .lua .modinfo, etc.. read their code, how they put it together, and then cut and paste (with permission) or at least get a feel for how the mod is laid out in code form.

One amazing tool for building combat based units (and some non combat) is modbuilder.jar by WHoward69 - you can find that utility at www.picknmixmods.com it has saved me countless hours of cutting and pasting for my own mod, and it will take any image file and turn it into an whole slew of unit icons for you (Automated) which is awesome .. unless you want custom art.

CivFanatics is the best place to get art, although some mods can be plagurized if permission is granted.