show only parent pages, then only show sub pages when in that section

Hi All,

Hope someone can help. I’m trying to create a main nav then a sub nav depending what page your on.

For example, parent pages:
– home
– about
– contact
– services

If you then click services, you would see a sub nav:
– marketing
– pr
– web
If you click any of these pages, you still see the other sub nav pages in this section.

How on earth do I achieve this?
TIA