From an experienced modder, first thing you need to do is play the game a few times all the way through and learn it. You say you want to mod as soon as possible, but what things do you want to mod?
Spend a couple days just opening files and looking at them. If you've no experience with modding or reading raw files it may take a few days to understand what's going on.
After you've learnt the game, and learnt the files, next you need try some simple things to ensure what you "think" the files do, they actually do. Change some names, add a new unit, a new faction, a new building. Get the hang of how the files inter-relate to each other.
Finally you get to come up with your own ideas and mod them in. 
