I am looking for what software can be used for changing some of the items in GECK. More or so, retexturing, or adding text to stuff in the game, or to armors.

There are a number of tutorials on this at fallout3nexus.com in the "mod talk" and "mod troubleshooting" forums in the pinned FAQ topics. Basically, to change textures, you need photoshop, or gimp or some equivalent. It is also helpful to have nifskope for that. To change meshes, you need nifskope.