Hi Elizabeth,
Which ERP system is it? I had earlier worked on a web service call to SAP. SAP team had build up a WSDL to handle any request from Ariba with input parameters, check/validations needs to be implemented in SAP and the return response back to Ariba. SAP team had implemented it in their SOA Manager and extracted a WSDL file with me which is a kind of simple XML file. I had processed the XML file to generate a java package. There are standard wsdl jar packages with which you can generate java package. We will need to identify which java class object we need to create to establish a connection with SAP. Write a custom java code, create an object of the web service connection, pass on username and password for authentication, and then pass required parameters through java code in your request and then handle the response.
Had used below apache packages for webservice call.
import org.apache.axis.client.Call;
import org.apache.axis.client.Service;
I hope you could get some idea of it.
Thanks,
Amit