use admin

db.createUser({user:"adminUser",pwd:"adminPassword", roles:["userAdminAnyDatabase"]})

or

db.createUser({
        user:"adminUser",
        pwd:"adminPassword",
        passwordDigestor:"server",
        roles:["dbOwner"]
    })

Add the following to /etc/mongod.conf or /usr/local/etc/mongod.conf

security:
    authorization: enabled

Restart mongodb server

To apply user with read write access to a database:

1. Switch to database you want to add user

   use kunal

2. Then apply role

   db.createUser(
   {
       user: "kunal",
       pwd: "kunal",
       roles:[
           {
               role: "readWrite",
               db:"kunal"
           }]
   })