Fixed menu

Hi WP team and friends!

I’m developing a new website using WP. I’m glad for using WP as a Cms!

My question is about the menu. My webpage has pages, not posts. This pages belong to each section, I mean:
Section 1: Page 11, Page 12, Page 13, …
Section 2: Page 11, Page 22, Page 23, …

So, I’d like to display the menu and avoid to click the word “Section 1”. I mean, I’d like that all the items show, like this:
http://www.informationarchitects.jp/en/downloads/

I wonder if there is a plugin or a free template that allow doing this.

Thanks a lot!!

andrea