on combo box selection shows content of selected item in listbox.

hi guys..
i am working on silverlight 4(c#,visual studion2010) using mvvm.
in my application i hav one combo box and listbox.
in combo box i am showing attribute name which is in employee attribute table.
every attribute name has his own table.
EG-in attribute name field has department.so department has his own table in database.
so wats i need when i select a item in combo box.then in list box shows a content of a selected item.
eg-suppose i select department then in listbox it should show the department table contents.
pls help how i do this.m not getting logic.
pls explain me with all code(viewmodel,xml,wcf service)
i hop dat i hav xplained enough so dat you all can help me in right direction.
pls solve my this problem as soon as possible.