Login in to the MongoDB server
$ mongo

use admin;

db.createUser(
  {
    user:"user1",
    pwd:"password",
    roles:[{role:"root",db:"admin"}]
    }
    )
exit

Connect to the MongoDb server using the user1 credentials
$ mongo -u user1 -p --authenticationdatabase admin
Enter the Password of the user that you have set and get connected to the mongoDb server.