Beren (sorry, but I've known you too long to start learning new names for you
), since you haven't been around here for too long (as a registered member anyway), I have no idea what you already know. So I'll start with from scratch.
The first thing you need is the Dom3 DB, which gives you all the unit numbers, stats, random magic masks and a lot of other stuff. The link is in my sig. Then you're going to need the sprite collection Kristoffer released. That can be found somewhere on the forum here.
The modding manual has a lot of the other stuff you need in some of the more obscure tables (flysprites, sound samples etc) since those have not been independently documented anywhere.
The more experienced modders here (Sombre, Foodstamp, Amos, Endoperez et al) can probably give you a lot of tips on how to optimize some things and get normally unmoddable things done (copystats features prominently in that). Endo is also a pretty good hand at recoloring and modifying the sprites from the game. So you can ask them for tips.
Be sure to check the modding related bug entries in the shortlist thread on the main forum like Sombre said, since that's really going to bite you in the description department. Hard limit is 183 descriptions as of the current situation and it is currently not possible to mod descriptions separately like weapons and then assign them to units. Which means that units with identical descriptions will still use up separate descriptions out of the total available.