sub pages as vertical menu (in left side of screen)

Hi,
I normally use this great template for designing static websites. Meaning that I normally strip away all blog features (categories, posts, side bars etc).

My question is this:
I wish to keep my main page menu at the top of the screen. But when clicking the menu "buttons" the sub menus "show up" as a vertical menu in the left side of the screen - where you would normally find the left sidebar.
I am sure this a really basic function but can't seem to find it! - Please help!

I am no good at code so please be patient!
Many thanks.,...

----------------------------
Menu1 Menu2 Menu3
----------------------------
submenu1 |
submenu2 | Content
submenu3 |