How to make the Menu items drop down to reveal more items

My website is located at photo.lums.edu.pk As you can see my main menu has the following items: Home, Portfolio, Shop, Events, Contact Us, and About. What I want is that when a visitor hovers over any of the above, a new drop down menu appears which reveals more items e.g. if a visitor hovers over Events a drop down menu appears with links to New Years Event, Birthday Events, Christmas Events etc. How can I add this feature?