Vertical_Drop_Down

I am setting up a website for the charity I work for using WordPress as a CMS, utilising nested pages to create the core structure. I need to implement a drop-down navigation menu that will contain selected pages and subpages and integrate neatly into my site as a whole.

If i add new page and select my portfolio page which will work as parent for the new page, now at the runtime when my mouse transfer (MOUSE OVER) to the portfolio page link, the pop up window will generate and the i want to see that new page in the vertical drop down menu. I hope what i am trying to say.