Do you have any experience with 3D modelling?
If not, Blender is a nice, fee, open source choice and you should get started here. You'll need to learn modelling, UV mapping, and how textures work.
Once you're comfortable with that, you should make a weapon, which is the easiest type of thing to get into the game. I recommend InsanitySorrow's tutorial here on TESA in the 3D craftworks section.
Architecture will require those skills, as well as creating collision. Collision creation is severely limited at the moment, but we can make static convex collision in nifskope; I think Hannaise used it to make her own custom house. Here's some info on that.
Hope this helps.