Creating object sitting on ground (e.g. text)

Hi all,

I want to place an object (2D or 3D) on the ground. So how can I 

- measure the current camera position (based on the sensor)

- get the real-world y compared to the current ground

- get the angle / current camera position and pose

- calculate all of the gathered data into something that I can use in libgdx for rendering the object correctly over the camera preview ?

What I am going to achieve: When walking through the streets, I want to see augmented objects lying there or some textual information.