How to create a main menu with dropdown?
You need to use any of this module

Nice Menus


  Nice Menus enables drop-down/right/left expandable menus. It uses only
  CSS for most browsers, with minimal Javascript for IE6.


Superfish Menu


  You will see several blocks called Superfish", move any of them to a
  region, use the "Configure" link of the block to configure it
  accordingly to your needs, and save.


Responsive Dropdown Menus


  
    Responsive Dropdown Menus provides a new menu tree rendering to make
    responsive, drop-down enabled main menu possible on any theme.
  

Provides new menu blocks for each menu.
All module blocks add dropdown menu support and responsive menu toggles.



Megamenu


  Mega menus are a type of drop-down, which seeks to overcome some of
  the usability drawbacks of traditional drop-downs. Hierarchy of your
  navigation is made visible all at once and the amount of clicking,
  searching, and navigating is reduced.