Going from 2D graphics to 3D?

Hi, I am trying to use some simple placeholders using 3d geometry.

Can anyone walk me through the steps of displaying them correctly?
I am trying to create a 3D version of my top down 2D display of a level I have built.
I do not know how to apply light to geometry so that my graphics can be seen properly.

I am using libGDX.

I understand how to get the graphics INto my game world. My problem is SHOWING them correctly.
I was using jMonkeyEngine, and my textures were always illuminated and I had nobody to show me how to make them dimmer, etc.