Do you want it to be a solid or to have 3D faces? either way it's not too complicated.
3D face would be an easy start so you can navigate around it and shade it etc.
Just start the command and pick your three corners of each face. You have four
of them.
To make it into a 3D solid, you can start with a rectangle solid that is your longest
length, width and height dimension. Then you want to create 2 triangle solids to
subtract from the rectangle solid to get your end result.
use plines and extrude to create your basic 3d shapes as required
or draw, modeling to get the basic shapes...
If you need more, ask away.
Pieter