Aside from models and textures, you're probably going to do almost all of your modding in the
TES Construction Set. You can find a lot of high-quality tutorials for it
here. If you plan on getting into it,
Blender is a popular free program for 3D modeling, and
GIMP is a popular free "painting" program for textures.
There are lots of easy mod-types to start with. House mods are a common starting point, retextured items
can be easy, or you could just fiddle with the various gameplay settings. It's said that you should mod for yourself before modding for the community.