The following Links gives you Step by step process about Webservice callouts using soap api and wsdl.

http://sriramp84.blogspot.in/2012/10/salesforceapex-callout-to-external.html

http://www.developer.com/net/net/salesforce-integration-with-.net-web-services-soap-api-.html

http://salesforce.stackexchange.com/questions/23101/wsdl-soap-ui-salesforce