javax.naming.InitialContext newInitialContext= new javax.naming.InitialContext();
is ther is any default ds to get the connetion ref..
which jndi name I have to use to get the Datasource
Assuming your question is about the portal since you posted in the Portal forum...the portal looks for a JNDI name of PortalDS.
Take a look in your jboss portal zip for a setup directory. Looks like you are using mysql so look for file portal-mysql5-ds.xml. Make adjustments to that file and drop it in the jboss server deploy directory.