For the moment based on your question I am assuming you want a single menu, which when hovering over a menu item, its child items are displayed in a drop down menu.
Following this tutorial should give you a horizontal menu with vertical drop downs:
http://codex.wordpress.org/Creating_Horizontal_Menus
Repeat for each menu you need.
For more on defining and using menus themselves in PHP, refer to this article by Justin Tadlock:
http://justintadlock.com/archives/2010/06/01/goodbye-headaches-hello-menus