“Use” command is use to create new database if database not exist or
switch to database if exist

USE  <database_name> 

> use cdac ;
switched to db cdac

But If you will check it will not show you newly created cdac database till you will not create any collection in new created cdac database

> show dbs;
admin 0.000GB
exampled 0.000GB
local 0.000GB
sachin 0.000GB

Create Collection and  check database is listed

> db.createCollection('student');
{ "ok" : 1 }

> show dbs;
admin 0.000GB
cdac 0.000GB
exampled 0.000GB
local 0.000GB
sachin 0.000GB

NEXT :CREATE COLLECTION