Go to www.3dbuzz.com, and sign up (it's free). Go back to the main screen, click Downloads at the top, then scroll down and click on the Doom 3 video tutorials. The first set deal with Doom 3's editor, and the second set deal with scripting. They don't cover true modding, but they're something to look into, at least.

Modding for a game engine is a time-consuming task. You won't find much in the way of material for beginners when it comes to modding, because most of it isn't for beginners. You need to know how the engine works, how the engine and the mod communicate, and many other things. I haven't looked into the Doom 3 engine much, but there will be detailed descriptions on the net. They will be technical. You'll need to have a very firm grounding in C/C++, and be prepared to invest quite a bit of time exploring how the code for a mod works. Don't expect to do a total conversion mod straight off the bat. I'd suggest sitting down with the Doom 3 game code, and trying to find the interface point between the engine and the mod. From there, you can usually follow code into the different subsystems. Once you get an idea of how things fit together, and what relies on what, you'll be able to start changing things. Use the internet sources. Experienced modders will have put a lot of time and effort into documentation and tutorials. And write lots of notes. A mod is a large chunk of code, and while you're exploring one part, you might forget some important things about a part you explored before.

Usually, there's a simple example of modding for an engine, to ease people into mod-making (the most basic mod you could make for Quake III, for example, was to change the rocket speed). Look for something like this. As with many things, start small, then move on to bigger things. Try here: http://www.iddevnet.com/doom3/.