Such things would be determined by the developer of the API that SoapUI is accessing.
SOAP is a rather standard and simple way to send a message from a client to a host. Think of each call as, for example, a web page. While ESPN.Com will accept your ESPN account, thinkgeek.com will not accept it. Without the correct information for the API (WebPage) that you are attempting to access we can not assist too much on things like acceptable inputs.
As for some basics on SOA:
W3Schools is always a great resource for tutorials on anything web based. I would also recommend
their WebServices training as a whole.
SoapUI Dojo has some resources about SOAP and such as well as some practices and tutorials.
Another thing that really stepped my game up in SOAP and webservices was to start developing my own web services. They are extremely easy to build, kinda fun to build and it really helps with the testing aspects of webservices in the sense that you learn how to build the web service, the client and test in between. Plus it is a lot of fun for me to do it. I often times build wrapper APIs around the API that I am testing in order to make it easier to test.
