The .NET Framework, in conjunction with Visual Studio has a facility to generate a proxy class for a WSDL-based SOAP web service. First, can you reference the WSDL of the service? You should be able to get to it with HTTP syntax like:
http://www.somesite.com/someservice?WSDL
That will differ from site to site, but if you can get to the WSDL, the tool in Visual Studio will be able to generate the proxy class.
Then, in Visual Studio, in your Solution Explorer, you should see an item labled "References". Right-click and a menu will appear, with an option for "Add Web Reference...". It is in here that you will use the URL to the WSDL of the site in question. Once the proxy class is generated, you should see methods in the class that correspond to the functions of the web service.