Side Bar Menu

I have a site where the menu’s will be on the side bar (left side)

I want to make the menus so that they will only display the sub pages when someone clicks on them, for example:
Before clicking:
Page1
Page2
Page3
Page4

When you are on a specific page.

Page1
Page2
–page 1/2
–page 2/2 (the person is on this page)
–page 3/2
Page3
Page4

How do I get this to show up on my site.