how you start is not important, and a hemisphere, if you're using subdivision modelling is perhaps not the best shape to start with.. but thats beside the point, now you have some geometry in your scene to build off, you need to start pushing vertexes around, extruding, cutting, welding and so on.. edit it a poly by poly to get the right shapes. Those terms might be different in c4d however.
Posts
http://www.scifi-meshes.com/forums/showthread.php?64877-Sub-D-Republic-Commando-Helmet&highlight=storm+trooper+helmet