How to trigger a event when there in any change in category in the service request

Hello Experts

                        I need to set Employee responsible dynamically when the exact category is chosen in the service request. i am following a approach to register a function module in the crmv_events so that the event will be called as soon as categorization is changed in service request.I have never configured crmv_events.please guide me its urgent.

Please tell me if a better approach is available.

I will award points immediately .Reply as early as possible as its urgent.

Thanks in advance