Guide to Modding
Beginners Guide
Fallout is really a very moddable game. It's not hard once you learn how to mod it either. The following are links to beginners guides which are very helpful:

(In order which I deem you should learn them)
Installing a Mod
Making a Mod
___________

Modding Tips

Advanced Guide
If you are familiar with hex and have had lots of experience with modding other games, then these guides are for you:

Item Offsets
Game File Descriptions
Drug Effects
Combat AI
Armor
Critters
Maps
DAT Specs
Converting BMPs to FRMs

Values
So, you've learned how to make a mod but are stuck on which value means what, this is what you need:

Armor Art Values
Inventory Art Values
Items List
Item Ground Frame Art List