Hello Kiran,
To Add Sub Menu Follow Below steps:

Go To Dashboard >> Appearance >> Menus
a. Create new menu
b. Give Menu Name
c. Select Menu items & drag Some menus Below Main Menu
d. Select menus as primary & click on save menu button.

https://luzuk.com/demo/nataraj-dance-studio/documentation/assets/images/menu1.jpg
https://luzuk.com/demo/nataraj-dance-studio/documentation/assets/images/menu2.jpg

Thank You.