how to drop db in mongo shell

// First show what databases have you gotten 
show dbs
// then, use the database name that you want to drop
use YOUR_DATABASE_NAME
// now you can drop it
db.dropDatabase()
// show collections should be empty now
show collectionsuse DB_NAME       // go to the db you want to delete

db.dropDatabase()

show dbs
// droppped db will not be shown