Selecting a sub category in a dynamically bind treeview and performing some operations

Sorry for asking silly question! I am new to WPF 

I am working with WPF treeview and binded it to database. It has Topics and sub-topics

for e.g : -Topic1

                ---Sub100

                ---Sub101

              -Topic2

                --Sub201

                --Sub202 and so on..

Now when I click on any sub topic it should  perform some operation according to my selection. How to do it?

How will I get selected data?

Thanks,

AJ