Your question is too open ended. I can answer specific questions, but that one reads to me like "how can I create a UI?", which is too open ended to answer.
All I can suggest is look at the examples that come with NGUI. Some of them involve 3D UIs. Any NGUI widget is just an object you can position like any other object in Unity. Try this for example:
1. New scene.
2. Create a game object and attach UIPanel to it.
3. Press ALT+SHIFT+L with that object selected. This will add a new label right under it. It will be huge -- so you will need to scale the panel object down.
There you go, an in-game UI element.
