Write Xml service for OFBIZ

[dharmendra.sahu08]

Hi

I am new in ofBiz,
I am working the services that are written in Xml.
like ProductServices.xml
Location /product/script/org/ofbiz/product/product/ProductServices.xml
That is written in xml.
please provide some suggestion.
how to write service in xml?
how to show the custom error message to the user.
how we customize the database error me

Thanks