Update #1

I have solved two main challenges that I need to clear.

1.How to render UI things

2.How to have the user interact with the UI (you talk about making buttons)

The way I render UI things is the same way, we did for rendering objects. The only difference is UI object will follow the camera rather than having it's own world coordination. (This is very inefficient way to render UI, so I might change it later)

For the user interact with UI, I just used win32.lib for windows. 

At this point, I have almost everything thing I need to make a button. The next things I am going to do is, make a delegate to bind a function to a button,  organize code to be user friendly. If I have more time I might also start working on changing button color when it's pressed, adding string to a a button as well