Listbox SelectedItem in MVVM

I'm trying to get my head wrapped around MVVM and have just run into a wall.  I have a ListBox with ItemsSource bound to a property on my viewmodel.  I need to have my viewmodel notified when the user selects an item in that listbox.  I've seen people posting SelectedItem="{Binding PropertyName, Mode=TwoWay}". 

Not sure what I'm doing wrong, but can anyone point me to an example of how something like this is accomplished within MVVM?

Thanks,
David