How do I make a clickable dropdown menu
There are several modules that can accomplish this, eg Superfish and Nice Menus.

If you want to roll your own, the basic steps are


Edit you menu items (admin/structure/menu/manage/menu-foo-menu) so that they are always expanded.  This will result in all menu items always being available, instead of only available in the active/sibling hierarchy.
Write some CSS and JS to get the menu behavior that you want.  With the way the nested menus are built in Drupal, you should be able to achieve a CSS only solution.  You can either do this with a custom module, or add it to your theme.  


The CSS and JS involved with this is beyond the scope of this site, but there are lots of resources on the net to help you out.