as mentioned you need the CS/GECK to make changes to the game - including adding new items. however for creating new items you will also need for both retexturing and modeling: a texture creating program (ie photoshop, GIMP etc.), and for creating new models Blender (or max/gmax/maya); also of course the blender(/max/maya) nifscripts for exporting to the right format and also good to have nifskope.
I would reccomend the tutorials/classes over at http://www.invision.tesalliance.org/forums/index.php?/index for teaching you how to model, texture and mod.
You'll also need NifSkope for the final adjustments to any model. It's not a modelling tool so much as an editor for the nif structure, and has lots of useful "fix-it" functions to make sure the results are compatible with the game.
For most now adays you don't need to to use nifskope at all (if using Blender)... of the top of my head the only things that require nifskope are such things as particle effects, some forms of morphing creature heads (other forms of morphs export fine and ready to use though)... even animated nifs don't require post-export nifskopeing now (as of last week's nifscripts release) if set up correctly in Blender.
Pacific Morrowind