Servlet for reading XML message

Greeting,
 
The company sent me an XML document (attached) that they say is the format their information will come to me in. I need to read that message & reply with additional tag
I don’t have high enough knowledge about this technology & I don’t ask total solution, hope to write Servlet for this but never try
 
What would be the best methods to use?

Please give me an advice/good example/link to start with this.

Thanks
dhanuxp
 
<?xml version=\"1.0\"?>\r\n")
 
 <bill_payment_request>
 
 <Authentication>
 <Client_ID>ABC</ Client _ID>
 < Client_password>ABC123</ Client _password>
 </Authentication>
 
 < BillpaymentsRequest >
 < Client _ID>XYZ</ Client _ID>
 <Location_ID>12</Location_ID>
 < Client_Transaction_ID>"+txnseq+"</Client_Transaction_ID>
<Amount>"+amount+"</Amount>
< Client _Transaction_DateTime>"+datetime+"</ Client_Transaction_DateTime>
<Cash_Credit>0</Cash_Credit>
 </BillpaymentsRequest>
 
< BillpaymentsRequest >
 < Client _ID>PQR</ Client _ID>
 <Location_ID>13</Location_ID>
 < Client_Transaction_ID>"+txnseq+"</Client_Transaction_ID>
<Amount>"+amount+"</Amount>
< Client _Transaction_DateTime>"+datetime+"</ Client _Transaction_DateTime>
<Cash_Credit>1</Cash_Credit>
 </BillpaymentsRequest>
 
 </bill_payment_request>