Kamailio default configuration file provides samples on how to configure a simple VoIP provider.

regarding SIP trunking it has examples for: user authentication and IP authorization.

In order to enable user authentication you need to define:

#!define WITH_MYSQL
#!define WITH_AUTH

This will enable MySQL DB back-end support and auth and auth_db modules. You should have MySQL server configured with a database required by kamailio and create subscribers with username, passwords.

After that you can register a SIP client with username/password previously created.