Lab - Creating First Application User

for creating first application user is it required to create "applicationData " database by our self?
if it is yes what collections and documents shoud i store in that database?

The db doesn’t need to exist. But if you want to test out the user, you can create a test collection. The next lab will utilise this new user and db.

if db doesnt exist means how it will create user in that db?

The only way to know is to try it out. After creating the user, run db.getUsers() to see the list of users on the admin db.

If we don’t have an existing db, you can simply create it using the command use db_name, db_name will be created internally, there is no extra command.