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
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Modding Tips
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
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