How to create Level1(primary) and Level2(secondary) Navigations ?

Sir,

I am developing a website in Drupal.

It has 4 primary navigations(level1 menus) on the top

When I click a primary navigation link, the corresponding level2 navigations should be displayed below that.

What should I do for that?

Ex: Menu1
menu1.1
menu1.2
menu1.3
menu1.4
menu1.5

ie when I click on the Menu1 link , all the five level2 navigatons(menu1.1, menu1.2, menu1.3, menu1.4, menu1.5) should be displayed below that.

How can I do this in Drupal?