It is really easy, but it is really hard.
You must take Archicad's 3D tools like the wall, roof and slab tools and BUILT A MODEL of the design using those elevations as guides.
Then you'll be free to extract renderings of the elevations and other views of the building using the model as the basis.
It is easy enough to take plain drawings into Archicad, but if you are going to place color and shadow on them by eye without benefit of modeling, you'll be better off to stay in your existing drafting program.
Start with the"getting started" found under the "help" menu and under this website's Support menu - the Archicad Wiki and the Interactive Training Guides.
http://www.graphisoft.com/support/archicad/documentation/
Dwight Atkinson