In general, I see more use of message-based (SOAP-DOC) web services than SOAP-RPC these days because of better interoperability. Either HTTP or HTTPS can be used depending on your needs. HTTP POST is recommended over GET.
Find (and read) the IS SOAP Programming Guide PDF. I think it is located in the \doc folder where Developer is installed. You should also work the SOAP examples contained in the WmSamples package.
Keep your first web services very simple by avoiding use of soap header variables or other more advanced features like soap over newer transports or soap with attachments. If you need those features you can always add them later after you gain some experience.
For external tools that you might need, you’ll want a tunneling utility that will allow you to see the messages as they are sent to/from your development server. You will probably also need an XML Schema design tool such as XML Spy or one of the zillion shareware tools.
Since you are new to web services and SOAP, you may also want to consider just having your trading partner send you an XML document using any supported method that both parties understand well (HTTP/S, FTP, email, 5.25" floppy, 9-track tape, etc.) 
Mark
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