How do I make a custom texture/mesh from scratch

I came across a problem: I am trying to make a cool sword, but I'm having a difficult time because every time I go to file> load on Nifskope, there are no files to load. I've got all of the meshes, textures and data unpacked correctly, but there is no data to open. How come? I'm also having the same problem with GIMP. (Every time I open up a file on GIMP, a pop-up says: opening file failed: unknown file type.) If someone could give me some help from there on making a custom texure, I'd be glad.