How to override existing behaviour in list,site....

You can follow the below URL to create and deploy a custom event handler

1. Create basic event handler
http://msdn.microsoft.com/en-us/library/ms437502.aspx

2. custom Event Handler Deployment using Feature..
You need two files.
-Feature.xml
-Elements.xml

http://msdn.microsoft.com/en-us/library/ms475328.aspx