I'm not sure why you mention Ruby on Rails, I'll assume that this has to
do with the provider of the web services for which you wish to write a
client. Therefore I'll ignore the Ruby on Rails part.

Plan A
------
This only uses the classes provided with the vanilla JDK.

Use HttpURLConnection and the Xerces SAXParser

http://www-128.ibm.com/developerworks/xml/library/x-soapcl/index.html
http://www.cafeconleche.org/books/xmljava/chapters/ch03s05.html

I find the above approach simple and effective. It helps that I know
what SOAP requests and responses look like on the wire (thanks to Perl,
SOAP::Lite, Wireshark and some 3rd party web-services)

Plan B
------
This requires you download lots of stuff and learn how to use it.

Download Apache SOAP, which requires you download JavaMail which
requires you download some other stuff. I thought "why do I need
JavaMail?" and went back to plan A.

Start here - http://ws.apache.org/soap/

I find the above approach daunting. Perhaps it looks easier after you've
climbed the mountain :-)

Get a web service toolkit and generate a stub from the WSDL.

Then calling the web service is just calling a method in a class.

Arne