Well I'm not much of a modder yet, but I started by adding simple items to the game, editing sprites, and figuring out the relations to the scripts and items.
Okay so others will tell you the parts of modding in terms of code and resources to make mods.
Make sure though that you have a PLAN of EXACTLY what you want to do before you start doing anything on a computer. Take it from an ex-programmer, if you don't have some sort of plan down before you start, you're just throwing stuff at a wall to see what sticks and it falls apart.
As for items, make sure you make a list of all the stats, perks affecting it, crafting, where it is in the world, loot tables and sprites. If this is all or mostly planned, you won't miss out on it later.