You need a consumer key and secret to connect to Twitter. Get one from dev.twitter.com/apps

1. Go to https://dev.twitter.com/apps/new and log in, if necessary

2. Supply the necessary required fields, accept the TOS, and solve the CAPTCHA.

3. Submit the form

4. Copy the consumer key (API key) and consumer secret from the screen into your application - classes/config.php:

e.g.
define('CONSUMER_KEY', 'My Consumer Key');
define('CONSUMER_SECRET', 'My Consumer Secret');
define('OAUTH_CALLBACK', http://www.example.com/callback.php');