Some themes (Display, Expose, Newscast, Avisio, CleanCut, Habitat) support the new Wordpress 3.0 Menu feature.
Just open Appearance->Menu in your backend sidebar and add a new Menu called "Main" (Newscast users must name the menus "First" and "Second"), then add pages, posts, categories, external links or whatever menu items you need to this menu. If you don't want to use the menu manager to create your own custom menu a fallback menu will get displayed that shows all pages and a home link.
Now before you add any menu items take a look at the top right of your browser window: you will find The "Howdy admin and Logout link" Directly below you can see the "screen options" tab. Click that tab and a panel with additional options will open. Check the checkbox in "Show advanced menu properties" that says "Description".
Now set up your menu. If you add menu items as children of other items you dont need to fill in the description. It wont get displayed. However top level items display the description directly bellow the Menu Item title in your front end.
Once an item is added to a menu, those menu items can be rearranged. Placing the mouse cursor over the menu item title, when the mouse cursor changes to 4-arrows, hold the left-mouse button down, drag the module to where you want to place it, then release the mouse button (this is called drag-and-drop). Remember you can drag a menu item slightly to the right of the menu item above it to create a hierarchy (parent/child) relationship in the menu.
Attention: You might need to delete some pre filled descriptions if you use pages in the second menu, since wordpress adds a content excerpt automatically to those items. Just remove that and enter a very short description of 2-7 words.
Attention: Newscast supports two menus so you can create two menus in your menu manager. Call the first menu (top menu with small links) "First" and the second menu (big menu items) "Second".