Hi! Don't know if this may be helpful, but when things start jittering around, it's often because there is some kind of collision happening, I guess it's your rigidbody colliding with the character controller.

You probably don't need the rigidbody (depending a little on what you want to create). Rigidbodies and character controllers are for the same purpose, handling collisions and applying physics. But they do this in different ways. I've had alot of problems with .fbx. I use maya (you can get a 6 month student license if you have a student-mail).

I think you may be best off looking through some of the tutorials and documents, before you dwelve to deep :D

If you know nothing about coding I always suggest having a look at the basics (like variables and syntax, if-statements, for-loops), and I would recommend javaScript for an easy start.

http://www.unitylabs.net/tutorials/unity-beginners/unity-scripting

you can make games with minimal code, but often you feel a little restricted.

If you want a more hands-on approach ( with some but minimal coding ), i suggest trying some tutorial.

There are loads of tutorials, and many of them deal in gravity, rigidbodies etc

Unity has some awesome tutorials here:

http://unity3d.com/support/resources/tutorials/

And unitylabs:

http://www.unitylabs.net/tutorials

Hope this helps! cheers