You lack permission to access the resource and method you requested.
All requests to sensitive areas of the Twilio API must use HTTP Basic Authentication. Authenticate using your Account SID as the username, and your Auth Token as the password. Both can be found in your Twilio Dashboard.
You may also want to consider using a Twilio helper library (available in PHP, .NET, Java, Ruby, and Python) as they will take care of authentication for you.