Event handling is a very broad term, please explain what it is your trying to achieve and the issue you are facing so that people can help you.
To create an eventhandler, select a control in the XPages source and there is a tab along the bottom in domino designer called "Events", this will allow you to add events and gives you many options such as partial refresh, partial execute, do not validate etc.
If you mean you want to handle errors in event handling, select an event handler and in the all properties tab there are 3 properties "onComplete", "onStart", "onError" that are available for you to execute code.
There are many examples of all of these on the web that you will find through google
such as: