Let me give you some advise out of personal experience regarding this.
A few things first, if you are in control of the web service I would recommend going for RESTful and returning and sending JSON data as parsing it using Objective-C is very easy.
There is a class library called SBJson you can use for the parsing of JSON data returned by the web service.
If you are not in control of what the web service will be returning and receiving, i.e. you didn't program the web service you can still use the SBJson classes for JSON data but I would recommend finding a XML library which you are comfortable in using.
http://www.raywenderlich.com is a great source of iOS tutorials with a in depth tutorial on XML parsing, which is used with SOAP.
The SBJSson library can be found at https://github.com/stig/json-framework/
Does these help?