By default, there is no authentication in MongoDb. But, you can create the equivalent of a root/superuser by using the "any" roles to a specific user to the admin database.

Something like this:

use admin
db.addUser( { user: "<username>",
          pwd: "<password>",
          roles: [ "userAdminAnyDatabase",
] } )

For version 2.6+

While there is a new root user in 2.6, you may find that it doesn't meet your needs, as it still has a few limitations:

For version 3.0+

Use db.createUser as db.addUser was removed.