If you have a model or waht to use one of VALVe's models you simply, in hammer select, prop_physics or prop_physics_multiplayer depending on what game you are going to use it for. If it is a brush hat you have made in hammer then you select the brush, press "to entity" and select func_physbox or func_physbox_multiplayer.
I'll need some more info from you to give a more detailed explanation..