Drupal 7 Bartik theme drop down menu
You can able to do it using Nice Menus module. Check here for detailed step by step tutorial on how to use nice menus module.


  Nice Menus enables drop-down/right/left expandable menus. It uses only
  CSS for most browsers, with minimal Javascript for IE6. (Version 2
  uses the Superfish jQuery plugin for all browsers, with an option to
  disable JS, and falls back to CSS-only for browsers that can handle
  it.)