If I were you, I would start looking through the neverwinter vault entries for custom content guides. I would also specifically download THE "Custom Content Guide v3". What you want is very specifically covered in that manual. I think this is the best place for anybody wanting to mod NWN to learn the basics. After that, we can discuss harder topics, like changing how many items you can associate with an item type, etc.
You'll find that the textures in NWN are either DDS or TGA. TGA easily converts to true BMP and imports into most graphics platforms. DDS can be unpacked into multiple sizes of TGA. Just be sure to export back to TGA with proper alpha channels and full color, or even go so far as to compile DDS before you release new content.
