3D Modelling of a ditch

[chintanp]

How should I model a random 3D-geometry using AUTOCAD CIVIL 3D?  

I have NEZ data from a total station survey of an uneven region. Importing them to AUTOCAD CIVIL 3D gives me points.    

What element should I use to model the 3D geometry enabling me to perform further calculations like volume etc. ?

[Neilw_05]

You will need to construct at least 2 surface models to compute volumes. It seems you need to learn a bit more about the software before attempting to use it. I recommend you at least go through the tutorials on surfaces, gradings and corridors. See the Help pull down to access the tutorials.