Answers for "mongodb shell create username and password"

mongo create user

db.createUser(
  { 
    user: "user_name", 
    pwd: "password", 
    roles: [
      "userAdminAnyDatabase", 
      "dbAdminAnyDatabase", 
      "readWriteAnyDatabase"
    ]
  }
)

mongodb create database with username and password

use mydb
db.createUser(
   {
     user: "myuser",
     pwd: "mypassword",
     roles: [ "dbOwner" ]
   }
)